-
第一课
Cytoplasm: The
Dynamic, Mobile Factory
细胞质：动力工厂
Most
of
the
properties
we
associate
with
life
are
properties
of
the
cytoplasm. Much
of
the
mass
of
a
cell
consists
of
this
semifluid
substance, which is bounded on the outside by the
plasma membrane. Organelles are suspended within
it, supported
by the filamentous
network of the cytoskeleton. Dissolved in the
cytoplasmic fluid are nutrients, ions, soluble
proteins, and
other materials needed
for cell functioning.
生命的大部分
特征表现在细胞质的特征上。细胞质大部分由半流体物质组成，并由细胞膜（原生质膜）包被。细胞器
悬浮在其中，并由丝状的细胞骨架支撑。细胞质中溶解了大量的营养物质，离子，可溶蛋白以及维持 细胞生理需求的
其它物质。
The
Nucleus: Information
Central
（细胞核：信息中心）
The eukaryotic cell nucleus is the
largest organelle and houses the genetic material
(DNA) on chromosomes. (In prokaryotes
the hereditary material is found in the
nucleoid.) The nucleus also contains one or two
organelles-the nucleoli-that play a role in
cell
division. A
pore-perforated
sac
called
the
nuclear
envelope
separates
the
nucleus
and
its
contents
from
the
cytoplasm. Small molecules can pass
through the nuclear envelope, but larger molecules
such as mRNA and ribosomes must
enter
and exit via the pores.
真核细胞
的细胞核是最大的细胞器，细胞核对染色体组有保护作用（原核细胞的遗传物质存在于拟核中）
< br>。细胞核含有
一或二个核仁，核仁促进细胞分裂。核膜贯穿许多小孔，小分子可以
自由通过核膜，而象
mRNA
和核糖体等大分子必
须通过核孔运输。
Organelles:
Specialized Work
Units
（细胞器：特殊的功能单位）
All eukaryotic cells contain most of
the various kinds of organelles, and each
organelle performs a specialized function in the
cell. Organelles
described
in
this
section
include
ribosomes,
the
endoplasmic
reticulum,
the
Golgi
complex,
vacuoles,
lysosomes,
mitochondria, and the plastids of plant
cells.
所有的真核细胞都含有多种细胞器，每个细胞器
都有其特定功能。本节主要介绍核糖体，内质网，高尔基体系，液泡，
溶酶体，线粒体和
植物细胞中的质体。
The number of
ribosomes within a cell may range from a few
hundred to many thousands. This quantity reflects
the fact that,
ribosomes are the sites
at which amino acids are assembled into proteins
for export or for use in cell processes. A
complete
ribosome is composed of one
larger and one smaller subunit. During protein
synthesis the two subunits move along a strand of
mRNA,
attached
to
a
single
mRNA
strand;
such
a
combination
is
called
a
polysome. Most
cellular
proteins
are
manufactured
on
ribosomes in the cytoplasm. Exportable
proteins and membrane proteins are usually made in
association with the endoplasmic
reticulum.
核糖体的数量
变化从几百到几千，核糖体是氨基酸组装成蛋白质的重要场所。完整的核糖体由大亚基和小亚基组成。
核糖体沿着
mRNA
移动并阅读遗传密码，
翻译成蛋白质。
一条
mRNA
上可能有多个核糖体，
称多聚核糖体。
大多数细
胞蛋白是由细胞质中核糖体生产。输出蛋白和膜蛋白通常与内质网有关。
The
endoplasmic
reticulum,
a
lacy
array
of
membranous
sacs,
tubules,
and
vesicles,
may
be
either
rough
(RER)
or
smooth
(SER). Both types play roles in the
synthesis and transport of proteins. The RER,
which is studded with polysomes, also seems
to be the source of the nuclear
envelope after a cell divides.
< br>内质网，带有花边的生物囊，有管状，泡状之分，以及光滑和粗糙面区别。两种都与蛋白质的合成和运输有 关。粗糙
内质网上分布许多核糖体，也可能提供细胞分裂后所需的细胞膜。
SER lacks polysomes; it is active
in the synthesis of fats and steroids and in the
oxidation of toxic substances in the cell. Both
types of endoplasmic reticulum serve as
compartments within the cell where specific
products can be isolated and subsequently
shunted to particular areas in or
outside the cell.
光滑内质网上无核糖体
，主要作用是脂肪和类固醇的合成以及细胞内有毒物质的氧化。两种内质网合成的产物在其中
进行分流或运输到细胞外。
Transport
vesicles may carry exportable molecules from the
endoplasmic reticulum to another membranous
organelle, the Golgi
complex. Within
the Golgi complex molecules are modified and
packaged for export out of the cell or for
delivery else where
in the
cytoplasm.
运输小泡能够将可运输分子从内质网运
输到高尔基复合体上。在高尔基复合体中修饰，包装后输出细胞或传递到细胞
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质中的其他场所。
Vacuoles
in
cells
appear
to
be
hollow
sacs
but
are
actually
filled
with
fluid
and
soluble
molecules. The
most
prominent
vacuoles
appear
in
plant
cells
and
serve
as
water
reservoirs
and
storage
sites
for
sugars
and
other
molecules. Vacuoles
in
animal cells carry out
phagocytosis (the intake of particulate matter)
and pinocytosis (vacuolar drinking).
细胞中的液泡好象是中空的，但实际上充满了液体和可溶分子。最典型的液泡存在于植物细胞
中，储备水，糖以及其
它分子。动物中的液泡起吞噬和胞饮作用。
A subset of vacuoles are the
organelles known as lysosomes, which contain
digestive enzymes (packaged in lysosomes in the
Golgi
complex)
that
can
break
down
most
biological
macromolecules.
They
act
to
digest
food
particles
and
to
degrade
damaged cell
parts.
溶酶体是液泡亚单位，含有消化酶，降解大部分
生物大分子。消化食物微粒和降解损伤的细胞残片。
Mitochondria are the sites of energy-
yielding chemical reactions in all cells. In
addition, plant cells contain plastids that
utilize
light energy to manufacture
carbohydrates in the process of photosynthesis.
It is on the large surface area provided by the
inner
cristae of mitochondria that ATP-
generating enzymes are located. Mitochondria are
self-replicating, and probably they are the
evolutionary descendants of what were
once free-living prokaryotes.
线粒体是细胞中化学产能的场所。另外，植物细胞中的质体在光合作用中利用光能产生碳水化合物，线粒体内 嵴上提
供了很大的表面积并分布着产
ATP
酶。线粒体自我复制，并且可能是自由生活的原核生物在进化中形成的后代。
There are two types of plastids:
leucoplasts, which lack pigments and serve as
storage sites for starch, proteins, and oils; and
chromoplasts,
which
contain
pigments. The
most
important
chromoplasts
are
chloroplasts-organelles
that
contain
the
chlorophyll used in photosynthesis.
The internal structure of chloroplasts includes
stacks of membranes called grana, which are
embedded in a matrix called the
stroma.
质体有两种类型：白色体，缺乏色素，是淀粉
，蛋白质和油的储备场所；色质体，含有色素。叶绿体是最重要的色质
体，含有与光合作
用有关的叶绿素。叶绿体的内部结构是由多层膜形成的叶绿体基粒，其中包埋在基质中的基粒称子
座。
The
Cytoskeleton
（细胞骨架）
All
eukaryotic
cells
have
a
cytoskeleton,
which
is
a
convoluted
latticework
of
filaments
and
tubules
that
appears
to
fill
all
available space in the
cell and provides support for various other
organelles. A large portion of the cytoskeleton
consists of
threadlike microfilaments
composed mainly of the contractile protein actin.
They are involved in many types of intracellular
movements in plant and animal cells. A
second protein, myosin, is involve in the
contraction of muscle cells. Another main
structural
component
of
the
cytoskeleton
consists
of
microtubules,
which
are
composed
of
the
globular
protein
tubulin
and
together
act
as
scaffolding
that
provides
a
stable
cell
shape. Cytoskeletal
intermediate
filaments
appear
to
impart
tensile
strength to the cell
cytoplasm. Mechanoenzymes such as myosin, dynein,
and kinesin interact with the cytoskeletal
filaments
and tubules to generate
forces that cause movements.
所有的细胞都有细胞骨架，网络结构的纤丝充满了它所能触及的全部空间并且对细胞器提供支持作用。细胞骨架
大部
分由微丝组成，微丝主要由可收缩的肌动蛋白组成。动植物细胞的许多种类型细胞内
运动与肌动蛋白有关。第二类蛋
白是肌球蛋白，它与肌肉细胞的收缩有关。细胞骨架的另
一个主要结构成分是微管，由球状的微管蛋白组成，象脚手
架一般维持细胞的稳定形态。
细胞骨架的中间丝提供了细胞质伸缩动力。机械酶，例如，肌球蛋白，动力蛋白，驱动
蛋
白与微丝，微管相互作用产生动力而引起细胞运动。
Cellular
Movements
（细胞运动）
Although the cytoskeleton provides some
stability to cells, its microtubules and filaments
and their associated proteins enable
cells to move by creeping or gliding.
Such movements require a solid substrate to which
the cell can adhere and can be guided
by the geometry of the surface. Some
cells also exhibit chemotaxis, the ability to move
toward or away from the source of a
diffusing chemical.
< br>尽管细胞骨架提供了细胞的某些稳定性，微丝，微管及相关蛋白能使细胞爬行或滑动。这种运动需要固体基 质依托并
通过表面几何形状的改变而运动。某些细胞具备趋药性，即趋向或逃离扩散开的
化学源。
Certain eukaryotic
cells can swim freely in liquid environments,
propelled by whiplike cilia or flagella. Both
cilia and flagella
have the same
internal structure: nine doublets (pairs of
microtubules) are arranged in a ring and extend
the length of the cilium
or flagellum,
and two more microtubules run down the center of
the ring. Every cilium or flagellum grows only
from the cell
surface where a basal
body is located. Movement is based on the
activities of tiny dynein side arms that extend
from one of the
microtubules of each
doublet.
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某些真核细胞能在液体液体中自由运动，由纤毛或鞭毛推
动。纤毛和鞭毛具有同样的内部结构：九个双微管环形排列，
纵向延伸，环中心是两个或
以上微管组成。纤毛或鞭毛从细胞表面的基体出生长，双微管的动力蛋白臂从一侧延伸到
另一侧而引起运动。
Nutrients,
proteins, and other materials within
most
plant cells are
moved about via cytoplasmic streaming.
The process
occurs as myosin proteins
attached to organelles push against microfilaments
arrayed throughout the cell. Microfilaments and
microtubules are responsible for almost
all
major cytoplasmic
movements. During cell division,
microtubules
of the spindle
assembled from tubutin subunits near
organelles called centrioles move the
chromosomes.
大部分植物细胞的营养，蛋白质和
其它物质由细胞质流运输。这个过程是由于依附在细胞器上的肌球蛋白反推排列在
细胞周
围的微丝形成的。绝大部分细胞质运动由微丝和微管完成。在细胞分裂期间，中心粒周围的由微管蛋白亚基装
配形成的纺锤体微管移向染色体。
第二课
Photosynthesis occurs only in the
chlorophyll-containing cells of green plants,
algae, and certain protists and bacteria.
Overall,
it is a process that converts
light energy into chemical energy that is stored
in the molecular bonds. From the point of view of
chemistry and energetics, it is the
opposite of cellular respiration. Whereas
cellular respiration is highly exergonic and
releases
energy, photosynthesis
requires energy and is highly
endergonic.
光合作用只发生在含有叶绿素的绿色
植物细胞，海藻，某些原生动物和细菌之中。总体来说，这是一个将光能转化成
化学能，
并将能量贮存在分子键中，从化学和动能学角度来看，它是细胞呼吸作用的对立面。细胞呼吸作用是高度放
能的，光合作用是需要能量并高吸能的过程。
Photosynthesis starts with CO2 and H2O
as raw materials and proceeds through two sets of
partial reactions. In the first set,
called
the
light-
dependent
reactions,
water
molecules
are
split
(oxidized),
02
is
released,
and
ATP
and
NADPH
are
formed.
These reactions must take place in the presence of
light energy. In the second set, called light-
independent reactions,
CO2 is reduced
(via the addition of H atoms) to carbohydrate.
These chemical events rely on the electron carrier
NADPH and
ATP generated by the first
set of reactions.
光合作用以二氧化碳和
水为原材料并经历两步化学反应。
第一步，
称光反应，
水分子分解，
氧分子释放，
ATP
和
NADPH
形成。此反应需要光能的存在。第二步
，称暗反应，二氧化碳被还原成碳水化合物，这步反应依赖电子载体
NADPH
以及第一步反应产生的
ATP
。
Both
sets
of
reactions
take
place
in
chloroplasts. Most
of
the
enzymes
and
pigments
for
the
lightdependent
reactions
are
embedded in the
thylakoid membrane of chloroplasts. The dark
reactions take place in the stroma.
两步反应都发生在叶绿体中。光反应需要的大部分酶和色素包埋在叶绿体的类囊体膜上。暗反应
发生在基质中。
How Light Energy
Reaches Photosynthetic
Cells
（光合细胞如何吸收光能的）
The
energy
in
light
photons
in
the
visible
part
of
the
spectrum
can
be
captured
by
biological
molecules
to
do
constructive
work. The
pigment chlorophyll in plant cells absorbs photons
within a particular absorption spectrums statement
of the amount
of light absorbed by
chlorophyll at different wavelengths. When light
is absorbed it alters the arrangement of electrons
in the
absorbing
molecule.
The
added
energy
of
the
photon
boosts
the
energy
condition
of
the
molecule
from
a
stable
state
to
a
less-stable
excited
state. During
the
light-dependent
reactions
of
photosynthesis,
as
the
absorbing
molecule
returns
to
the
ground
state, the
生物分子能捕获可见光谱中的光能。
植物细胞中叶绿素在不同光波下吸收部分吸收光谱。在吸收分子中，光的作用使
分子中的
电子发生重排。光子的能量激活了分子的能量状态，使其从稳定态进入不稳定的激活态。
All photosynthetic organisms contain
various classes of chlorophylls and one or more
carotenoid (accessory) pigments that also
contribute to photosynthesis. Groups
of pigment molecules called antenna complexes are
present on thylakoids. Light striking
any one of the pigment molecules is
funneled to a special chlorophyll a molecule,
termed a reaction-center chlorophyll, which
directly participates in
photosynthesis. Most photosynthetic organisms
possess two types of reaction-center chlorophylls,
P680
and
P700,
each
associated
with
an
electron
acceptor
molecule
and
an
electron
donor. These
aggregations
are
known
respectively as photosystem
Ⅰ
(P700) and
photosystem
Ⅱ
(P680).
所有的光合作用生物
含有不同等级的叶绿素和一个或多个类胡萝卜素（光合作用的辅助色素）
。称作天线复合
体的色素
分子群存在于类囊体中。激活色素分子的光能进入叶绿素反应中心，其直接参与
光合作用。大部分光反应细胞器拥有
两套反应中心，
P680<
/p>
和
P700
，每个光系统都含有一个电子
受体和电子供体。这些集合体就是大家熟识的光合系统Ⅰ和
光合系统Ⅱ。
The Light-Dependent Reaction:
Converting Solar Energy into Chemical-Bond
Energy
3
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20
光反应：光能转化成化学键能
The photosystems of the light-dependent
reactions are responsible for the packaging of
light energy in the chemical compounds
ATP and NADPH. This packaging takes
place through a series of oxidation reduction
reactions set in motion when light strikes
the P680 reaction center in photosystem
Ⅱ
. In this initial event
water molecules are cleaved, oxygen is released,
and electrons
are donated. These
electrons are accepted first by plastoquinone and
then by a series of carriers as they descend an
electron
transport chain. For each
four electrons that pass down the chain, two ATPs
are formed. The last acceptor in the chain is the
P700
reaction
center
of
photosystem
Ⅰ
.
At
this
point
incoming
photons
boost
the
energy
of
the
electrons,
and
they
are
accepted
by
ferredoxin. Ferredoxin
is
then
reoxidized,
and
the
coenzyme
NADP+
is
reduced
to
the
NADPH.
The
ATP
generated previously
and the NADPH then take part in the light
independent reactions.
光反应的光
系统将光能转化成化学复合物
ATP
和
NADPH
。当光激活光系统Ⅱ的光反应中心时，通过一系列的氧化还
< br>原反应实现能量的传递。反应开始时，水被分解，氧被释放并提供电子。电子首先传递给质体醌，然后通过 一系列载
体形成的电子传递链。每传递
4
个电子，形成
2
个
ATP
。最后一个受体存在于光反应系统Ⅰ的反应中心里。此处光子
激活电子，<
/p>
电子传递给铁氧还蛋白。
铁氧还蛋白再氧化，
并且辅酶
NADP
+
还原成
NADPH
。
早期产生的
ATP
和
NADPH
进入暗反应。
The
production
of
ATP
from
the
transport
of
electrons
excited
by
light
energy
down
an
electron
transport
chain
is
termed
photophosphorylation. The one-way flow
of electrons through photosystems II and I is
called noncyclic photophosphorylation;
plants also derive additional ATP
through cyclic photophosphorylation, in which some
electrons are shunted back through the
electron transport chain between
photosystems
Ⅱ
and
Ⅰ
.
由电子传递链偶连产生
ATP
的过程称
为光合磷酸化。通过光合系统Ⅱ流经光合系统Ⅰ的电子路径称非循环式光合磷酸
化；植物
通过循环式光合磷酸化获得额外的
ATP
，一些电子在光合系统
Ⅰ和Ⅱ之间的电子传递链中回流。
The Light-
Independent Reactions: Building
Carbohydrates
暗反应：碳水化合物的形成
In
the
light-independent
reactions
of
photosynthesis,
which
are
driven
by
ATP
and
NADPH,
C02
is
converted
to
carbohydrate. The
reactions are also known as the Calvin-Benson
cycle. Atmospheric CO2, is fixed as it reacts
with ribulose
biphosphate (RuBP), a
reaction that is catalyzed by the enzyme ribulose
biphosphate carboxylase. The reduction Of C02 to
carbohydrate (fructose diphosphate) is
completed via several more steps of the cycle.
Finally, RUBP is regenerated so that the
cycle may continue.
< br>由
ATP
和
NADPH
驱动的暗反应中，二氧化碳转化成碳水化合物。即卡尔文循环。二磷酸核酮糖固定二氧化碳，由二
磷酸核酮糖羧化酶催化。
Oxygen: An Inhibitor of
photosynthesis
（氧：光合作用的抑制因子）
High levels of oxygen in plant cells
can disrupt photosynthesis and can also cause
photorespiration-an inefficient fun of the
dark reactions in which 02 is fixed
rather than C02 and no carbohydrate is
produced.
Reprieve from
Photorespiration: The C4 Pathway
Most
plants
are
C3
plants;
they
experience
decreased
carbohydrate
production
under
hot,
dry
conditions
as
a
result
of
the
effects of photorespiration. Among C4
plants, however, special leaf anatomy and a unique
biochemical pathway enable the
plant to
thrive in and conditions. Thus C4 plants lessen
photorespiration by carrying out photosynthesis
only in cells that are
insulated from
high levels of CO2. They also possess a novel
mechanism for carbon fixation.
< br>大部分植物是碳
3
植物，在高温干旱条件下，由于光呼吸
作用而使碳水化合物的合成降低。而在大多数的碳
4
植物中，<
/p>
由于叶脉的特殊构造和独特的化学路径使植物依然很茂盛。这是碳固定的一个新机制。
第三课
The
Nucleus and Chromosomes
The
cell nucleus is the main repository of genetic
information. Within the nucleus are the
chromosomes tightly coiled strands
of
DNA
and
clusters
of associated
proteins.
Long
stretches
of
the
continuous DNA
molecule
wind
around
these
clusters
of
proteins,
or
histones,
forming
beadlike
complexes
known
as
nucleosomes.
More
coiling
and
supercoiling
produces
a
dense
chromosome structure. Each long strand
of DNA combines with histones and nonhistone
proteins to make up the substance
chromatin.
细胞核是贮藏
遗传信息的主要场所。
DNA
盘绕成螺旋线以及相关的成簇蛋白
质。
DNA
螺旋线缠绕成簇的组蛋白形成
珠链状的核小体。
这些螺旋和超螺旋形成致密的染色体组结构。
每个长链
DNA
与组蛋白和非组蛋白一起构成染色质物
质。
4
/
20
A pictorial display of an
organism's chromosomes in the coiled, condensed
state is known as a karyotype. Karyotype reveal
that in most cells all but sex
chromosomes are present as two copies, referred to
as homologous pairs. Non-sex chromosomes
are
called
autosomes. Organisms
whose
cells
contain
two
sets
of
parental
chromosomes
are
called
diploid;
those
with
cells
containing a single set of parental
chromosomes are called haploid.
染色体致密的超螺旋状态我们称染色体组。除了性染色体外，大多数细胞的染色体组成对出现，称同源染
色体对。非性染色
体称常染色体。生物细胞含有两套父母本染色体的称二倍体；含有单套
染色体的称单倍体。
The Cell
Cycle
The cell cycle is a
regular sequence in which the cell grows, prepares
for division, and divides to form two daughter
cells, each
of which then repeats the
cycle. Such cycling in effect makes single-celled
organisms immortal. Many cells in multicellular
organisms, including animal muscle and
nerve cells, either slow the cycle or break out of
it altogether.
在细胞生长过程中，细胞循环
遵循特定程序，分裂准备，分裂成
2
个子细胞，子细胞再循环。
此循环使得单细胞永生。
多细胞生物中的许多细胞，包括动物肌肉和神经细胞，要么降低
循环速度，要么同时分裂。
The normal cell
cycle consists of four phases. The first three
include G1, the period of normal metabolism; S
phase, during
which normal synthesis of
biological molecules continues, DNA is
replicated, and histones are synthesized; and G2,
a brief
period of metabolism and
additional growth. Together the G1, S, and G2
phases are called interphase. The fourth phase of
the
cell
cycle
is
M
phase,
the
period
of
mitosis,
during
which
the
replicated
chromosomes
condense
and
move
and
the
cell
divides. It is believed that
properties of the cell cytoplasm control the cell
cycle, along with external stimulators and
inhibitors
such as chalones.
正常细胞循环由
4
个时期组成。
头三期包括
G1,
正常新陈代谢；
S
期
,
正常新陈代谢同时
，
DNA
复制，
组蛋白合成；
G2
期，短期的新陈代谢和少许生长。
G1,
S,
和
G2
称分裂间期。最后是
M
期，有丝分裂期，复制的染色体组浓缩，
移动
并细胞分裂。据称是染色质控制了细胞循环，伴随外部激活因子和抑制因子如抑素。
Mitosis: Partitioning the
Hereditary Material
Biologists divide the mitotic cycle
into four phases. At the beginning of prophase
the chromosomes each consist of two highly
condensed
chromatids
attached
to
each
other
at
a
centromere. As
prophase
ends
and
metaphase
begins,
the
condensed
chromosomes
become
associated
with
the
spindle. Eventually
the
chromosomes
become
arranged
in
a
plane
(called
the
metaphase plate) at a right angle to
the spindle fibers. Next, during anaphase, the
two sister chromatids of each chromosome
split, and one from each pair is drawn
toward each pole of the cell. During telophase
nuclear envelopes begin to form around
each set of chromosomes, and division
of the cytoplasm takes place.
生物学家将有丝分裂划分为
4
个阶段。分裂前期，高度浓缩
的两个染色单体通过着丝粒连接在一起。在分裂前期后期
和分裂中期前期，浓缩的染色体
与纺锤体相连，最后以正确的角度排列在赤道板上。在分裂后期，两个姊妹单体分离，
分
别拽向细胞两极。在分裂末期，在每套染色体周围形成核膜，细胞质发生分裂。
As mitosis proceeds, the spindle
microtubules play a crucial role in ensuring that
both paired and separated chromatids move in
the right directions at the proper
times. Each half of the spindle forms as
microtubules extend from each pole of a dividing
cell
to
the
region of
the
metaphase
plate. During
prophase,
other
microtubules,
the
centromeric
fibers,
extend
outward
from
the
spindle
poles
to
structures
on
the
chromosomes
called
kinetochores. During
anaphase
the
fibers
begin
to
shorten,
and
the
chromatids begin to move
apart.
在有丝分裂过程中，是纺锤体微管确保了染色单
体在适当时间以正确方向进行分离。纺锤体微管由两极向赤道板延伸。
在分裂前期，其它
微管，着丝粒纤维延伸到染色体的动粒。在分裂后期，纤维开始变短，染色单体分离。
The spindle forms differently in plant
and animal cells. In animals it is associated
with centriole, while in plant and fungal cells
spindle formation is associated with
reions called microtubule organizing
centers.
植物和动物细胞形成的纺锤体不同。动物细
胞与中心粒相连，而在植物和真菌细胞中，纺锤体与微管组织中心的离子相连。
Cytokinesis: Partitioning the
Cytoplasm
胞质分裂：细胞质分离
The
division of the cell cytoplasm at the end of
mitosis is called cytokinesis. In animal cells it
takes place as a ring of actin
filaments
contracts
around
the
cell
equator,
pinching
the
cell
in
two. In
plant
cells,
which
are
bounded
by
a
cell
wall,
cytokinesis involves the building of a
new cell plate across the dividing cell at its
equator. Cell wall material is then deposited
in the region of the cell
plate.
在动物细胞中，环形肌动蛋白丝延赤道板收缩而
使细胞一分为二。在植物细胞中，在赤道板形成新的细胞板。
Meiosis: The Basis of Sexual
Reproduction
5
/
20
Meiosis is a special form
of cell division that takes place in the
reproductive organs that produce sex cells. Like
mitosis, it
takes
place
after
DNA
replication
has
occurred
and
involves
two
sequential
nuclear
divisions
(meiosis
I
and
meiosis
Ⅱ
). These
divisions
result
in
four
daughter
cells,
each
with
half
the
number
of
chromosomes
of
the
parent
cell.
The
phenomenon of crossing over during
meiosis results in exchanges of genetic
information between chromosomes. Hence, the
homologous chromosomes distributed to
different progeny cells are not
identical.
减数分裂是性细胞分裂的特殊形式。<
/p>
如有丝分裂，
它也是发生在
DNA
复制后并有连续的两个核分裂。
产生
4
个子细胞，
分别含有亲本一半的染色体数。
As in mitosis two chromatids exist for
each chromosome at the beginning of prophase 1.
During this phase the homologous
chromosomes
undergo
synapsis,
or
pairing,
which
is
brought
about
by
a
bridging
structure
of
proteins
and
RNA
called
the
synaptonemal complex. The homologous
pairs stay together when they align on the
metaphase plate. Unlike the anaphase of
mitosis, however, during anaphase I the
two chromatids of each chromosome stay joined at
the centromere and move together
to one
of the two poles of the cell. It is this event
that results in the halving of the chromosome
number in the four daughter
cells that
result from meiosis.
正如在有丝分裂
中一样，两个同源染色单体通过蛋白质和
RNA
桥配对形成联会
复合体。与有丝分裂不同的是，每组染
色体的两个染色单体连接在着丝点上并一起移向细
胞两极的一级。由此而导致
4
个子细胞染色体数减半。
During
telophase
I
nuclear
envelopes
enclose
the
chromosomes
in
nuclei,
and
in
most
species
cytokinesis
(the
first
nuclear
division)
follows. The
second
nuclear
division
begins with
metaphase
Ⅱ
,
in
which
the
chromosomes
in
each
daughter cell
again align on a metaphase plate. The
centromeres finally divide, and each sister
chromatid moves to one of the poles of the
spindle. The next phase is telophase ,
followed again by cytokinesis. The result of the
entire process is four haploid cells in
which parental chromosomes are randomly
distributed.
第二次核分裂开始于分裂中期，子
细胞中染色体重新排列在赤道板上。着丝粒最终分离，每个姊妹染色单体分向两极。
接着
胞质分裂。产生
4
个单倍体，父母染色体随机分配。
Asexual Versus Sexual
Reproduction
Mitosis
and
meiosis,
respectively,
make
simple
cell
division
and
sexual
reproduction
possible. Each
means
of
passing
on
hereditary
information
has
advantages. In
asexual
reproduction
the
parent
organism
gives
rise
to
offspring
that
are
genetic
clones
of
the
parent. The
advantages
of
this
type
of
reproduction
are
that
it
preserves
the
parent's
successful
genetic
complement, requires
little or no specialization of reproductive
organs, and is more rapid than sexual
reproduction. A major
disadvantage of
the asexual mode is that a single catastrophic
event or disease may destroy an entire population
of genetically
identical organisms. A
prime benefit of sexual reproduction is that it
provides genetic variability and a ready mechanism
for the
elimination of deleterious
mutations. It also allows
有丝分裂和减数分裂在传递遗传信息过程中各有优势。体细胞的繁殖就是父母本的克隆，其优势是保留了父母本的
成功遗传
信息，不需要特殊器官，比性复制快的多。但一个简单灾难性事件或疾病都可能
摧毁一个细胞群体。性复制的优势是它提供
了遗传可变性和现存排除有害突变的机制。也
可以产生新的基因并在种群中蔓延。
第四课
Foundations
of Genetics
Early Theories of
inheritanee
Early ideas of inheritance
included Hippocrates' theory of
pangenesis
and August
Weismands
germ
plasm theory.
Based
on experiments with
mice, Weismann proposed that hereditary
information in gametes transmitted traits to
progeny.
Both of
these early views incorporated the
blending theory: they held that heritable traits
of the two parents blend, so that the distinct
characteristics of each are lost in
offspring.
遗传学的早期理论包括泛生说和种
质理论。基于小鼠实验，维丝曼提出遗传信息储存在配子中并将遗传信息传递给后代。
这
两个早期观点合起来形成融合理论：子代拥有父母本混合的遗传特征，而不完全象亲代。
Gregor Mendel and the Birth of Genetics
Gregor Mendel, an Augustinian monk in
the monastery at Brunn, Austria, is known as the
exposed
to
theories
of
the
particulate
nature
of
matter
while
a
university
student
and
having
a
background
in
mathematics,
Mendel
carried
out
a
series
of
carefully
planned
experiments
that
demonstrated
the
particulate
nature
of
heredity.
His
revolutionary ideas were
neither understood nor accepted until many years
after Mendel died.
孟德尔，众所周知
的遗传学之父，是一名修道士。当他还是大学生时就提出了物质的粒子属性。孟德尔进行了一系列周密
安排的实验来证实遗传的颗粒性。直到他去世后，他的理论才被理解和接受。
6
/
20
Mendel's Classic Experiments
Mendel studied genetics through plant-
breeding experiments with the garden pea, a plant
species that is self-fertilizing and
breeds true (each offspring is
identical to the parent in the trait of interest).
To test the blending
theory, he focused his
research on
seven distinct characters.
Each of these characters, such as seed
color and plant height, present only two, clear-
cut
possibilities.
He also recorded the type and number of
all progeny produced from each pair of parent pea
plants, and followed
the results of
each cross for two generations.
孟德尔通过豌豆
实验研究遗传学，豌豆是自花授粉植物和纯品系。为验证融合理论，他的研究主要集中在
7
个特征上。例
如，种子颜色，植株高度，这些特征只有两个明
确的可能性。他记录了产生的每一个子代类型和数量，在杂交产生子
2
< br>代。
For each of the
characters he studied, Mendel found that one trait
was
dominant
while the other
was
recessive.
In the second
filial
(F
2
)
generation,
the
ratio
of
dominant
to
recessive
was
3:1.
Mendel
deduced
that
this
result
was
possible
only
if
each
individual
possesses
only
two
hereditary
units,
one
from
each
parent.
The
units
Mendel
hypothesized
are
today
known
as
alleles,
alternative forms
of genes.
Genes
are the
basic units of heredity. An organism that inherits
identical alleles for a trait
from
each
parent
is
said
to
be
homozygous
for
that
trait;
if
different
alleles
for
a
trait
are
inherited,
the
organism
is
heterozygous
for that trait.
When an organism is heterozygous for a trait, the
resulting phenotype for that trait expresses only
the dominant , the organism’s
phenotype
—
its
physical appearance and properties-
differs from its
genotype,
which
may include both a
dominant and a recessive allele.
A pictorial representation of all
possible combinations of a genetic cross
is known as a
Punnett
square.
对于每个特征而言，要么显形，要么隐性。在
子
2
代中显形与隐性比为
3
∶
1
。只有在每个个体仅拥有两个研究遗传单元，
并每个单元来自一个亲代时，实验结果才成立。此遗传单元就是今天共识的等位基因。两
个一样的等位基因决定一个特征，
称纯合。相反，称杂合。当生物是杂合时，它的表型由
显性基因决定。因此，生物的表型与基因型是不同的。旁纳特方格可
以陈列所有可能的遗
传组合。
The
results
of
Mendel's
experiments
on
dominant
and
recessive
inheritance
let
to
Mendel's
first
law:
the
law
of
segregation.
This law states
that for a given trait an organism inherits one
allele from each parent.
Together these alleles form
the allele pair. When gametes are
formed during meiosis, the two alleles become
separated (halving of chromosome number).To
gain
evidence
for
his
theory
Mendel
performed
test
crosses,
mating
plants
of
unknown
genotype
to
plants
that
were
homozygous recessive for the trait of
interest.
The ratio of
dominant phenotypes (if any) in the progeny makes
clear whether
the unknown genotype is
heterozygous, homozygous dominant, or homozygous
recessive.
分离定律，生物只遗传父母本等
位基因对的一个等位基因。减数分裂期形成配子时两个等位基因分离。为验证此理论，
他
做了测交实验，即基因型未知的植物与纯合的隐性基因植物杂交。子代显性表型可以明确测得杂合基因或纯合基因
的基因
型。
Mendel's
Ideas and the Law of independent Assortment
Mendel also performed
dihybrid crosses,
which
enabled him to consider how two traits are
inherited relative to one another.
This work let to the
law of
independent assortment,
which states
that the alleles of genes governing different
characters are
inherited
independently.
An
apparent
exception
to
Mendel's
laws
is
incomplete
dominance,
a
phenomenon
in
which
offspring of a cross
exhibit a phenotype that is intermediate between
those of the parents.
However, incomplete dominance
reflects
the
fact
that
both
alleles
for
the
trait
in
question
exert
an
effect
on
the
phenotype.
The
alleles
themselves
remain
separate.
双因子杂合试
验，两个特征是如何相互影响遗传的。试验结果产生独自分配定律，即等位基因独立遗传。特例是，不完
全显性。子代的表型是父母本的中间类型。不完全显性说明了两个等位基因对表型都有影响。
，等位基因会继续分离。
Mendel
presented his ideas in 1866 in a scientific paper
published by the Brunn Society for Natural
History.
Unfortunately,
the meaning of his research was not
understood by other scientists of the day.
His work was rediscovered
in 1900 by Carl
Correns and Hugo de
Vries.
1866
年，孟德尔在自然史上发表了他的
科学论文，陈诉了他的观点。不幸的是，他的研究不被当时科学家接受。在
1900
年，他的著作再被发现利用。
Chromosomes and Mendelian Genetics
Soon after Mendel's work was
rediscovered, Walter Sutton and Theodor Boveri
independently proposed that the hereditary
units might be located on chromosomes.
Experiments to prove this
hypothesis were carried out by Thomas Hunt Morgan
and
his students at Columbia
University, in research on the sex chromosomes of
fruit flies.
Morgan's
studies were also the first
7
/
20
exploration
of sex-linked traits.
It
also led to the discovery in 1916 by Calvin
Bridges of the phenomenon of
nondisjunction,
in which a
chromosome pair fails to segregate during meiosis.
孟德尔著作被再发现不久，
Walter Sutton
和
Theodor Boveri
提出
，遗传单位可能定位在染色体组上。伴性遗传又导致了
不分离现象的发现，即在减数分裂
中，染色体对不分离。
第五课
Genes Code for Particular
Proteins
The
first
scientist
to
investigate
the
question
of
how
genes
affect
phenotype
was
Sir
Archibald
Garrod,
whose
studies
of
alkaptonuria implied a relationship
between genes and enzymes. Thirty years later
Beadle and Ephrussi showed a relationship
between
particular
genes
and
biosynthetic
reactions
responsible
for
eye
color
in
fruit
flies.
Next,
in
a
series
of
classic
experiments
on
the
effects
of
mutations
in
the
bread
mold
Neurospora
crassa,
Beadle
and
Tatum
explored
the
one-
gene-one-enzyme hypothesis-the idea that each gene
codes for a particular enzyme. Their work paved
the way for other
researchers to
elucidate the precise ways in which enzymes affect
complex metabolic 1949, in research on the role
of
hemoglobin
in
sickle
cell
anemia,
Linus
Pauling
helped
refine
the
one-
gene-one-enzyme
hypothesis
into
the
one-
gene-one-polypeptide hypothesis.
Archibald
Garrod
是第一个研究基因是如何影响表型的科学家，他对尿黑酸症的研究揭示了基因与酶之间的关系。
Beadle
和
Ephrussi
< br>在三十年后对果蝇眼睛颜色的研究发现特殊基因与相关反应的生物合成有关。接着对面包发霉粗糙
脉孢菌的突变试验得出一个基因一个酶的假说。他们的工作为其他工作者铺平了道路，即精确地阐
明了酶影响了复杂
的新陈代谢途径。在
1949
年，对镰刀状细胞贫血症的研究对一个基因一个酶的假说进一步上升为一个基因一个多肽。
The Search for the Chemistry and
Molecular Structure of nucleic Acids
Nuclei acid, originally isolated by
Johann Miescher in 1871, was identified as a prime
constituent of chromosomes through the
use
of
the
red-staining
method
developed
by
Feulgen
in
the
early
1900s. Frederick
Griffith's
experiments
with
the R
and S
stains of pneumococci showed that an as
yet unknown material from one set of bacterial
could alter the physical traits of a
second set. In the 1940s the team of
Avery, MacLeod, and McCarty showed that this
unknown material was DNA. At about
the
same
time
P.A.
Levene
discovered
that
DNA
contained
four
nitrogenous
bases,
each
of
which
was
attached
to
a
sugar
molecule
and a phosphate group-a combination Levene termed
a nucleotide.
在
18
71
年，核酸最初是由
Johann
Miescher
分离成功，并由
Feulgen
在
1900
年证实核酸是染色体组最基本的组成。<
/p>
Frederick Griffith
对粗糙和光滑的肺炎球菌
实验表明，
不确定的某种物质可以从一组细菌转移到另一种细菌中。
在
1940
年，确认该物质为
DN
A
。四个碱基和磷酸分子分别连接在糖分子上，称核苷酸。
Disagreement over whether DNA could
carry complex genetic information was ended in the
early 1950s by Martha Chase and
Alfred
Hershey, whose work with E. coli showed clearly
that DNA, and not protein, is the bearer of
genetic information.
直到
1950
年，通过对大肠杆菌实验发现，遗传物质是
DNA
，而不是蛋白质。
Each
DNA nucleotide contains a five-carbon sugar,
deoxyribose, attached to one of four bases:
adenine, guanine, cytosine, or
thymine.
Adenine and guanine molecules are double-ring
structures called purines, while cytosine and
thymine are single-ring
structures
called pyrimidines. The molecule made up of a
base plus a sugar is termed a nucleoside. In each
molecule of DNA a
phosphate group links
the five-carbon sugar of one nucleoside to the
five-carbon sugar of the next nucleoside in the
chain. This
phosphate bonding creates
a sugar-phosphate backbone.
每个核苷酸都含有一个五碳脱氧核糖，分别连接
4
个碱基，即：
腺嘌呤，鸟嘌呤，胞嘧啶，胸腺嘧啶。碱基连接糖称
核苷。磷酸键形成磷酸骨架。
Chargaff’s rules describe the
fact that (1) the amount of adenine is equal to
the amou
nt of thymine in DNA, with
amount of
cytosine equal to that of
guanine, and (2) the ratios of A to T and of C to
G vary with different species.
< br>（
1
）腺嘌呤与胸腺嘧啶，胞嘧啶与鸟嘌呤相等；
（
2
）腺嘌呤与胸腺嘧啶，胞嘧啶与鸟嘌呤的
比例随物种不同而不同。
The Research
Race for the Molecular Structure of DNA
In the late 1940s and early 1950s,
researchers looking for the structure of DNA drew
upon Chargaff s insight, Levene's ideas on
DNA components, and two other lines of
evidence. One was the suggestion of Linus Pauling
that DNA might have a helical
structure
held in place by hydrogen bonds, and the other was
X-ray diffraction photos of DNA, showing a helical
structure with
distance between the
coils, taken by Franklin and Wilkins.
<
/p>
直到
40
年代末
50
年代初，
研究者在寻求
DNA
结构过程中，
确立了
Chargaff
的观点和
, Levene
的组成理论
以及其他两
个线索。
一个是
Linus
Pauling
的假设，
DNA
可能具
有螺旋结构，
通过氢键连接。
另一个是
X-
衍射图片，
Franklin and
Wilkins
提供。
8
/
20
Based on
this information Watson and Crick proposed the
double helix model of DNA-A twisted ladder-like
molecule with two
outer sugar phosphate
chains and rungs formed by nucleotide pairs.
Paired nucleotides, which always occur as A-T or
G-C, are
linked by hydrogen bonds.
Watson and Crick also proposed that genetic
information is encoded by the sequence of base
pairs
along the DNA
molecule.
基于这些信息，
Watson
和
Crick
提出了双螺旋结构模型，成对的核苷酸通过氢键相连，遗传信息就贮藏在碱基对中。
How DNA Replicates
In their model of DNA structure and
function, Watson and Crick hypothesized that DNA
replicates itself by
the
hydrogen
bonds
joining
A
to
T
and
C
to
G
.
This
process
would
produce
two
opposite
halves
that
could
then
serve
as
templates
for
the
construction
of
new,
complementary
strands. This
model
of
semiconservative
replication
conservative
because each new molecule has one half
of the former parent molecule-was later confirmed
by the work of
DNA
进行复制是以拉链方式自我复制，产生的两个二分体分别为模板生成互补链，即半保留复制。并由
Meselson
和
Stahl
验证。
In E. coli DNA replication begins with
the formation of a bubblelike structure on the
circular chromosome that is produced by
replication forks. Studies of
bacterial DNA replication have shown that a
growing DNA chain lengthens only in the 5' to 3'
direction (from the 5' carbon of one
sugar to the 3' carbon of the next). The leading
strand is synthesized continuously, while
the
lagging
strand
is
synthesized
in
short
stretches
known
as
Okazaki
fragments. The
enzyme
DNA
polymerase
links
free
nucleotides as they line up on the
template formed by the original strand of the
parent molecule.
In
eukaryotes DNA replication follows the same
general principles as in prokarotes. On the long
DNA molecules replication
proceeds (in
two directions at once) from hundreds or thousands
of points of origin.
大肠杆菌复制开
始时形成泡样复制叉，链生长方向由
5
′
向
3
′
端，前导链连续生成，后随链
由冈崎片段组成，由
DNA
聚
合酶催化
。真核生物复制与原核生物复制相似，但有几百到几千个复制原点（原核一般只有一个复制原点）
。
第六课
A Home for
Life: Formation of the Solar System and Planet
Earth
The story of life's
origins begins with the formation of the earth.
The sequence of events that gave rise to our
planet began, in
turn, with the cosmic
explosion physicists call the Big Bang. The sun
at the center of our solar system condensed from a
cloud
of primordial matter roughly 5
billion years ago; the planets, including the
earth, condensed about 4.6 billion years ago. The
earth is composed of a number of
layers: a solid crust, a semisolid mantle, and a
largely molten (liquid) core that has a solid
center. Basic
physical
features
of
Earth
that
may
have
made
the
emergence
of
life
possible
include
the
planet's
size,
temperature,
composition, and distance from the sun. The major
current hypothesis holds that life arose
spontaneously on the
early earth by
means of chemical evolution from nonliving
substances.
生命起源于地球的形成。大爆炸是我
们行星形成的开始。太阳在
50
亿年前生成，位于太阳系中心，
行星，包括地球，
在
46
亿年前生成。
地球由多层组成：坚硬的地壳，半流体的地幔，一个很大的溶解中心中存在一个坚实的中心。地球
的基本特征使生命起源成为可能，包括行星的大小，温度，组成以及离太阳的距离。当前主要假设认为，
由非生命的
化学物质进化过程自发产生了生命。
The Emergence of life: Organic and
Biological Molecules on a Primitive
Planet
Evidence
for
prelife
stages
of
chemical
organization
comes
from
laboratory
experiments
that
try
to
duplicate
the
physical
environment and
chemical resources of the early earth. These
experiments, including the pioneering work of
Miller and Urey,
have successfully
produced organic monomers including amino acids,
simple sugars, and nucleic acid bases. The
probable next
step toward life was the
spontaneous linking of such monomers into polymers
such as proteinoids and nuclei acids. Current
research suggests that likely sites for
this polymerization were clay or rock
surfaces.
通过创造出地球早期的自然条件和化学资
源条件，科学家在实验室中已经获得了化学有机体生命前阶段的证据。这些
实验包括米勒
等早期所做工作，成功地产出了有机单体，包括氨基酸，单糖，核酸碱基。这些单体自发的连接成多聚
体，如类蛋白和核酸，使进入生命状态成为可能。这些聚合作用可能发生在泥土或岩石表面。
Researchers
have
found
that,
when
energy
is
available
to
a
system,
they
can
generate
three
kinds
of
organic
molecular
aggregates. The
Russian
Aleksandr
Oparin
obtained
polymer-rich
droplets,
called
coacervates
from
solutions
of
polymers. Sidney Fox generated
proteinoid microspheres from mixtures of amino
acids and water. A third laboratory structure
is the liposome, a spherical lipid
bilayer that forms from phospholipids. A
structure similar to one or more of these
aggregates
may have been the precursor
of true cells.
9
/
20
当一个系统获得能量时，可
能发生
3
种有机分子的聚集。从多聚物的溶液中俄国科学家获得
了富含小液滴的多聚物，
即凝聚物。从氨基酸和水的混合液中
F
ox
获得了类蛋白微球体。实验室第三个结构物质是脂质体，即由磷脂形成的球
形脂双层结构。
Further
steps
in
the
appearance
of
cells
on
the
earth
included
the
development
of
RNA
and DNA
as
biological
information
molecules. Evidence suggests that RNA,
which can form spontaneously under conditions
mimicking those of the early earth,
was
the first informational molecule. The discovery
of RNA ribozymes-RNA that can act as an enzymelike
catalyst suggests
that such catalytic
RNA also could have assembled new RNAs from early
nucleotides. Certain catalytic RNAs can also
carry
out sexlike exchanges of pieces
of RNA.
RNA
可能是地球早
期形成的第一个贮藏信息的物质。在实验室模拟早期地球自然条件下其可自发形成。核酶的发现说
明它可以催化早期核酸形成新的
RNA
。对于
RNA
物种而言，某些催化
RNA
具备类似于性交换的功能。
Following
the development of a lipid-protein surface layer
and replicating RNA and DNA informational
molecules, the events
leading to the
emergence of living cells would have included the
origin of the genetic code; the sequestering of
RNA or DNA
into cell-like structures;
and the development of metabolic
pathways.
接下来脂蛋白表层的形成，
RNA
的复制
,DNA
信息
分子的形成，最终导致活细胞的出现，包括最初的遗传密码，
RNA
或
DNA
被包裹进细胞样的结构中；及代谢途径的建立。<
/p>
The Earliest
Cells
The oldest fossils
that may represent living cells are found in rocks
that are about 3.5 billion years old. The cells
were probably
anaerobic heterotrophs,
with autotrophs arising much later. The first
autotrophs produced their own nutrients and
released O,-a
metabolic by-product that
had a crucial impact on later life forms. The
resulting ozone layer in the earth's atmosphere
reduced
the penetration of ultraviolet
light. As a result, cells would survive in
shallow water and on the land surface. The
increasing
quantity of atmospheric
oxygen also permitted the evolution of aerobic
cells and cellular respiration, which in turn
signaled the
beginning of the global
carbon gh the earliest cells were all prokaryotes,
by about 1.5 billion years ago eukaryotes
appeared.
能说明活细胞存
在的最古老化石大约有
35
亿年了。最早出现的细胞可能是厌氧
异氧生物，自养生物很久后出现。最早
的自养生物自己生产营养并释放氧气，这个新陈代
谢副产品对后期生命的形成有一个深远的影响。臭氧层的出现减少
了紫外线的渗透。结果
，细胞就能够在浅水区和陆地上生存了。大气中氧气数量的增加使得需氧细胞进化并产生细胞
呼吸，这预示着全球碳循环的开始。最早期的细胞都是原核生物，直到
15
亿年后，真核细胞才出现。
The
Changing Face of planet Earth
Changes in land masses, the seas, and
climate have greatly affected the evolution of
life on the earth. The basic parts of the
planet include a light, solid crust
over a hot, semisolid mantle and an inner,
partially molten core. Massive segments or plates
of
the
crust
move
over
the
mantle
in
the
process
of
continental
drift. Over
the
past
500
million
years,
continental
drift
has
sculpted the earth's
crusts to produce the form and distribution of
present-day continents. Climatic changes that
greatly affected
living organisms
accompanied these plate movements; the period was
marked by occasional waves of mass extinctions of
living
creatures. Organisms were also
affected by periods of glaciation that followed
variations in the earth's orbit and in the output
of
energy by the sun.
大陆板快，海洋的改变，以及气候对地球生命的进化都有深远影响。地幔上大板快地壳的挤压形成大陆
漂移。大陆漂
移雕塑了地壳的外观，使现在大陆形成。伴随板块运动，气候改变对活有机
体有深远影响。在特定时期的生物大量灭
亡高峰是这个时期的见证。生物也受冰河期影响
，在冰河期，地球轨道和太阳能的输出都发生了很大变化。
Taxonomy: Categorizing the Variety of
living Things
Biologists use
the binomial system of nomenclature developed by
Linnaeus to categorize the varieties of life on
the earth. The
system
assigns
each
type
of
organism
to
a
genus
and
species. Organisms
are
then
further
classified
into
higher
taxonomic
categories-family, order, class,
division (plants), phylum (animals), and kingdom.
Evidence from many subfields of biology,
such
as
biochemistry
and
comparative
anatomy,
helps
define
species
and
higher
taxa
(taxon). And
whereas
species
were
originally
defined
in
terms
of
morphological
traits,
today
biologists
generally
use
the
criterion
of
a
reproductively
isolated
population.
生物学家利
用林奈发展的双名法对生物分类。系统选定每个类型的生物进入属和种，然后将生物进一步分类更高级类
别中，即科，目，纲，门，界。来自于生物化学和比较解剖学等亚生物学领域的证据有助于划分物
种和更高级的分类
单位，然而物种最初依据形态学特征进行分类的，今天生物学家大体上
使用孤立多产的群体作为标准。
Taxonomy
reveals
a
great
deal
about
the
evolutionary
relationships
among
organisms.
A
clade
is
a
taxonomic
unit
whose
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20
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