Transcript 小孢子母细胞

Ch .2 Mitosis and Meiosis
有丝分裂与减数分裂
Genetic continuity between cells and organisms
of any sexually reproducing species is
maintained by the processes of mitosis and
meiosis . The processes are orderly and efficient,
serving to produce diploid somatic cells and
haploid gametes,respectively. It is during these
division stages that the genetic material is
condensed into discrete,visible structures called
chromosomes.
Chapter
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content
1.cell structure
2.homologous chromosome, haploidy,diploidy
3.mitosis and meiosis
4.cell cycle control
5.meiosis and reproduction
6.spermatogenesis and oogenesis
7.The significance of meiosis
8.The relationship between chromatin and
chromosomes
Vocabulary Terms
• Chromatin
• nucleolus
• NORs (nucleolar organizer
regions
• centromere
• kinetochore
• Homologous chromosomes
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Locus
karyokinesis
cytokinesis
cell cycle (all phases)
sister chromatids
microtubules
molecular motors
apoptosis
1.cell structure
The nucleolus
 nucleoli: nuclear
organelles containing
rRNA
 nucleolar organizers:
genes encoding
rRNA
The most prominent substructure within the nucleus
during interphase is the nucleolus. 100 years ago, one found
a big spherical structure,called nucleolus within the nucleus,
but up until 1960s, it was not identified that the nucleolus is
a ribosome production factory, designed to fulfill the need
for large scale transcription and processing of rRNA and
assembly of ribosomal subunits.
r RNA genes and their transcription and processing
The nucleolus is organized around the chromosomal
regions that contain the genes for the 5.8S, 18S and 28S
rRNA within a single transcription unit for a 45S pre-rRNA,
which is transcribed within the nucleolus by RNA
polymerase I. The 5S RNA, another component of 60S (large)
ribosomal subunit is transcribed outside of the nucleolus by
RNA polymerase III.
Eukaryotic cells contain multiple copies of rRNA
genes (or called rDNA) to support the production of a
large numbers of ribosomes. For example, the human
genome contains about 200 copies of the gene
encoding the single transcription unit for 5.8S, 18S
and 28S rRNAs, and approximately 2000 copies of
the gene encoding 5S rRNA. The genes for 18S, 5.8S
and 28S rRNAs are distributed into 10 clusters in
tandem array near the tip of one of the two copies of
five different chromosomes (13, 14, 15, 21, and 22);
the 5S rRNA genes locate in a single tandem array on
chromosome 1..
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Another typical example is Xenopus
oocytes, rRNA genes of mature oocytes (in
diameter 1mm) are amplified about 2000fold, resulting in ~1million copies of rRNA
gene and nearly 1012 ribosomes per cell.
The cells need take ~500 years to produce
such huge numbers of ribosome, if the
oocytes were without the selective
amplification of rRNA genes
CHROMOSOME STRUCTURE
1: nucleosomes, 10 nm = DNA + histone beads
octomer (2×): H2A, H2B, H3 & H4
2: solonoid, 30 nm = coil stabilized by histone H1
3: solonoid loops attached to scaffold protein by
scaffold attachment regions (SARs) on DNA
4: supercoil, ~700 nm: = chromatid at cell division
染色体的结构
一、染色质的基本结构
染色质（chromatin)—染色体在细胞分裂
间期所表现的形态,呈纤细的丝状结构，
也称为染色质线。
原核生物——裸露的环状双链DNA，通常只有
一个。2000年测定发现霍乱菌（Vibrio
cholerace)有两个环：
大的——2961kb
小的——1072kb
染色体的组成成分
染色体=DNA+组蛋白—DNA.蛋白质纤丝重复折叠
而成
DNA
1
 染色体
蛋白质=组蛋白 1 （5种） +非组蛋白
少量RNA
 Kernberg(1974)提出染色体绳珠模型：
DNA.蛋白质纤丝基本结构单位——核小体
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核心颗粒8个组蛋白分子（H2A、H2B、H3、H4)—140bp + H1-60bp
Nuclesome
NDA双链环绕核心外1.3/4,140bp
（200bp 11nm)
连接区50-60个碱基+组蛋白
此时染色体长度压缩了7倍（ 一级结构）
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CHROMOSOME STRUCTURE
CHROMOSOME STRUCTURE
一级结构
CHROMOSOME STRUCTURE
真核生物染色体
1、常染色质：染色体的主要成分。染色较
浅，着色均匀，分裂间期，常染色质程
高度分散状态，占据核内大部空间。与
分裂状态相比，折叠凝缩包装较为松散。
 常染色质的凝缩状态与基因的活性有关。
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具有活性的基因一定在常染色质中，但常染色质
中的基因并非全部处于活性表达状态。通常只有
一部分基因进行转录
2、异染色质
 折叠非常致密、染色较深。分裂周期中致密程度
很少改变。
 间期细胞核内染色很深的异染色质程簇状分布在
核膜和核仁四周。位于异染色质中的基因没有转
录活性。
 根据异染色质DNA序列的不同又可分为
 （1）组成型异染色质 一种永久性的在染色体有
固定位置的异染色质，着丝粒周围，由不表达的
DNA序列（卫星DNA）组成，稳定染色体结构
的作用
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组成型异染色质大多为染色体的一个区段，也
有组成整条染色体。如果蝇的Y染色体。某些
动物除正常染色体（A染色体）之外，还有一
些数目不等的额外染色体（B染色体），称为
超染色体，多为异染色质构成。
（2）功能型异染色质
又X异染色质，特定条件下由常染色质转变而
来。例如哺乳动物的X染色体。雌性个体细胞
中有两个X染色体，其中一条随机失活，处于
异染色质状态，而另一条有活性的X染色体仍
是常染色质。
染色体的结构模型
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Kernberg把核小体—核小体通过连接区（60bp)以
一定间隔连接形成一串珠子，称为绳珠模型。
研究得知，所谓的绳珠模型是在制备染色体时
DNA上的H1丢失（或被破坏）的结果。
在活体细胞中，核小体与核小体贴近的 ，由不同
H1相互作用，核小体卷曲盘旋呈中空状的螺线
管——外径30nm、内径10nm，相邻的螺旋间距
11nm、每一周螺旋6个核小体，此时又压缩了6倍。
螺线管进一步螺旋化——超螺线管，直径40nm的
二级结构
圆筒状结构-超螺旋体此时又压缩40倍。
超螺线管进一步螺旋或折叠——形成有丝分裂所见
三级结构
到的染色体，此时又压缩5倍。
8400倍=7 X 6 X 40 X 5倍。
四级结构
染色质环的结构
30nm纤维
(30nm fiber)
三级结构
300nm
核基质
(Nuclear Matrix)
The Organization of 30nm fiber into chromosomal loops
CHROMOSOME STRUCTURE
2.homologous chromosome,
haploidy,diploidy
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The number and shape of
chromosomes vary from species to
species
levels of organization:
1. ploidy – chromosome “sets”
2. n – how many of each “type”
3. size – somewhat arbitrary
4. position of centromere, arm length
5. landmarks – chromomeres
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CHROMOSOME karyotpye
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human chromosome set: 23 pairs
Grouping
Banding
2. 4 .Chromosome classification and karyotype
Chromosomes are identifiable based on the position of
the centromere
Chromosome classification
Type
中间着丝粒
metacentric
long/short arm symbol
1-1.7
division behavior
M.m
V
近中着丝粒
1.7-3
submetacentric chro
Sm
L
近端着丝粒
acrocentric chro
3-7
St
l
顶端着丝粒
acrocentric chro
7—
T.t
chro
l
Karyotype
Well-stained
metaphase spreads
Photographed
Each of chromosome
images is cut out of
the picture
Matched with its
partner Arranged from
largest
to smallest on a chart
The largest autosome
is number 1
G-banding - Giemsa stain
R-banding - reverse banding
C-banding
Q-banding
Fluorescent banding
Banded chromosome (give differential
staining along the length of chromosome)
 Intercalating agent: staining compound that
insert between the base-pairs of DNA
 Quinacrine: a fluorescent compound (are
detected only when DNA are exposed to
Ultraviolet). Quinacrine that have inserted into
the chromosome to emit energy. Parts of the
chromosome shine brightly, whereas other
parts remain dark.
 The Staining procedure is called: Q-banding
 The Bands that it produces are called: Q-band
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The banding patterns depend on staining procedures and
the extent of chromosomal condensation.
1.Q-banding: Stain with Quinacrine or similar fluorescent
dye view by UV fluorescence
Types of staining:
2. G-banding: Pretreat with trypsin, stain with Giemsa
3. R-banding: Heat-denature then stain with Giemsa.
4. C-banding: Denature with saturated Ba(OH)2 then
stain with Giemsa.
Idiogram
Morphological characteristics of
chromosomes - Size
CHROMOSOME TOPOGRAPHY
 Drosophila
chromosomes
 centromeres
 telomeres (later)
 euchromatin
 heterochromatin
 polytene chromosomes
CHROMOSOME TOPOGRAPHY
3.mitosis and meiosis
• Review the process of mitosis, and
observe the 4 phases of mitosis
•Review the process of meiosis, and
observe the various phases of meiosis
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Cell division and Cell Cycle: the events that occur
from the completion of one round of division to
the beginning of the next
The field of developmental genetics investigates the
genetic basis of the changes in form that an
organism passes through during its life cycle. One
cellular process that is common throughout these
changes in form is cell division. The two cell
division events that need to be controlled are the
entry into the S-phase when DNA is replicated, and
the entry into the M-phase when mitosis occurs. In
this regard, two timing events need to be monitored
by the cell. These are
1. when to initiate replication (S-phase entry)
2. when to begin chromosomal condensations (Mphase entry)
Most of the Cell Cycle is spent in interphase
4.Cell Cycle regulation
(control)
The cell cycle ,including both mitosis and miosis ,is
fundamentally the same in all eukaryotic organisms.
 The similarity of the event leading to cell duplication
in various organisms indicated that :
Governed by genetic program
Conserved throughout evolution .
 Disruption of this regulation may lead to the
uncontrolled cell division , which is related with the
cancer.
 Many genes control the cell cycle
 Mutations cdc(cell division cycle)that interrupt the cell
cycle are funded
 The study of these mutations has established at least
three major checkpoints exist.
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Regulation of the Eukaryotic cell cycle
Two critical events:
 1. Nuclear DNA replication
 2. Mitosis (cell division)
 are fundamentally similar in all eukaryotic
cells
 Master controller of these events -- heterodimeric protein kinase
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regulatory subunits
catalytic subunit
Postmitotic
cells:
“exit” the cell
cycle
G0
Restriction
point
the point in late
G1
where passage
through
the cell cycle
become
independent of
mitogens
•Cyclins and Cdk Cyclin-dependent kinase
proteins regulate cycle
•G1/S -check for damage from last division
•G2 /M-check for damage during replication
•M -check for proper spindle fiber formation
•p53 gene causes apoptosis at G1 checkpoint
Cell cycle
A variety of genes and proteins cotroll the
event of cell cycle. These genes and
proteins allow progression to next stage of
the cycles when all is well, but cause to
brake when damage to the genome.
 Cyclin-dependent kinase collaborate with
cyclin to ensure proper time and sequence
of cell cycle events.
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70 cell cycle genes are found
CDKs
CDC28 from saccharomyces cerevisiae
CDC 2
schizsaccharomyces cerevisiae
CDK4
CDK2
 CYCLINS
D
E
A
B
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G1 to S phase
G2 to M phase
Mitosis yields
two identical
diploid cells
5.Meiosis and sexual
reproduction
Meiosis yields
four daughter
cells that are
not identical
Meiotic prophase I is further subdivided into 5 substages
•Leptonema-Condensation & Homology search begin
•Zygonema- Homology search complete, bivalents form
•Pachynema-Synapsis occurs, crossover occurs
•Diplonema- Chiasma evident
•Diakinesis- Chiasma move toward end of tetrad
(terminalization)
•Metapase I- chiasma
still evident,
homologues align
randomly across
equator
•Anaphase Ihomologues separate
•Telophase I- variable,
from none to short
interphase
•Note that cells are
now haploid
2nd Meiotic
Division
ensures each
daughter cell
receives a
single
chromatid
from original
tetrad
Oogenesis occurs in the
ovaries and produces
one haploid egg
•May continue throughout
life, all year long
•May be periodic in some
species
•Process often arrested in
prophase I (e.g. humans)
•resumes years later just prior
to ovulation
Spermatogenesis occurs in
the testis and produces 4
haploid sperm
动物生活史
卵子形成
(Oogenesis)
性原细胞
（gonia)
卵原细胞
(oogonia)
次级母细胞
第一极体
次级卵母细胞
(First polar body)
（Secondary oocyte)
卵细胞
(ovum)
第二极体
(Second polar body)
受精卵
(oosperm)
(Spermatogenesis)
精原细胞
(spermatogonia)
初级母细
胞
I
初级卵母细胞
(Primary oocyte)
精子形成
II
配子
(gametes)
合子
(zygote)
胚胎
(embryo)
初级精母细胞
(Primary spermatocyte)
次级精母细胞
(Secondary
spermatocyte)
精细胞
（Spermatids）
精子
（spermatozoa)
动物个体
Multicellular plants alternate between haploid and
diploid generations
Plant life cycle
Megasporogenesis
雄花
Microsporogenesis
雌花
MMC大孢子母细胞(2n)
Microspore mother
小孢子母细胞(2n)
meiosis
meiosis
Endosperm (3N)
大孢子(n)
极核
卵核
The (N) embryo sac
胚囊(n)
originally had
8 (N) nuclei
but two fused together so
the functional mature
embryo sac (female
种子
胚乳(3n)
胚(2n)
Microspore小孢子
Pollen花粉
Mature Pollen成熟花粉
精核(n)
发芽花粉
A mature pollen grain
has three (N) nuclei [2
sperm nuclei + 1 Tube
nucleus].
In the embryo sac, there are three (N)
antipodal nuclei, two (N) synergid nuclei,
one (N) egg nucleus, and two (N) polar
nuclei, which fused together to form one (2N)
polar nucleus.
一个雄核+卵核——二倍体2n——胚
Double
Fertilization 一个雄核+极核——三倍体3n——胚乳
1 Sperm Nucleus
(N)
+
1 Fused Polar
Nucleus (2N)
=
Endosperm (3N)
[lining of
endosperm is
aleurone,
which is also 3N]
1 Sperm Nucleus
(N) + 1 Egg Nucleus
(N)
=
Zygote (2N)
Mitosis
Embryo (2N)
Mitosis
Mature Adult Plant
(2N)
7.The significance of Meiosis
Functional Significance of Meiosis is
Increased Genetic Variability While Ensuring
Genetic Constancy Between Generations
Sexual Sources of Genetic
Variation
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For asexually reproducing organisms, the only
source of genetic variation is mutations.
For sexually reproducing organisms, there are
three additional mechanisms for generating
genetic variation:
1. Independent assortment of chromosomes.
2n Combinations of Maternal and paternal
Chromosomes
2. Crossing Over
3. Random fertilization.
Independent Assortment of
Chromosomes
Homologous chromosome pairs are oriented
at random along the metaphase plate during
meiosis I.
 Each chromosome of a homologous pair has
an equal probability of ending up in one or
the other daughter cell.
 For N homologous pairs of chromosomes,
there are 2N possible types of gametes.
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For examples:
For Drosophila melanogaster, there are 4
homologous pairs of chromosomes (2N = 8);
therefore, there are 16 possible types of
gametes (24).
 For Homo sapiens (humans), there are 23
homologous pairs of chromosomes (2N =
46); therefore, there are 8,388,608 possible
types of gametes (223).
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Independent Assortment of
Chromosomes - continued
Type 1
Type 2
Type 3
Type 4
Random Fertilization
During fertilization, haploid gametes from
two genetically different individuals are
combined to form a diploid zygote.
 If each parent can produce a total of X
different types of gametes, then the total
possible number of genetically unique
zygotes is X2.
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For example
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, ignoring crossing over, Drosophila
melanogaster can produce 24 = 16 different
gametes through the independent
assortment of chromosomes. Therefore,
each male and female fly can produce 24 x
24 = 28 = 256 genetically unique
combinations of zygotes.
For example
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ignoring crossing over, humans can
produce 223 = 8,388,608 different gametes
through the independent assortment of
chromosomes. Therefore, each male and
female human can produce 223 x 223 = 246 =
70,368,744,177,664 genetically unique
combinations of zygotes.
8.The relationship between
Chromatin and Chromosome
Each chromosome consists of one
long strand of DNA, along with its
associated proteins. This complex of
DNA and protein is called
“chromatin”.
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