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Ch .2 Mitosis and Me iosis 有有有有有有有有有 Genetic continuity between cells Genetic continuity between cells and organisms of any sexually and organisms of any sexually reproducing species is reproducing species is maintained by the processes of maintained by the processes of mitosis and meiosis . The mitosis and meiosis . The processes are orderly and processes are orderly and efficient, serving to produce efficient, serving to produce diploid somatic cells and diploid somatic cells and haploid gametes,respectively. It haploid gametes,respectively. It is during these division stages is during these division stages

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Page 1: 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

Ch .2 Mitosis and Meiosis 有丝分裂与减数分裂

Genetic continuity between cells and organisms Genetic continuity between cells and organisms of any sexually reproducing species is of any sexually reproducing species is maintained by the processes of mitosis and maintained by the processes of mitosis and meiosis . The processes are orderly and meiosis . The processes are orderly and efficient, serving to produce diploid somatic efficient, serving to produce diploid somatic cells and haploid gametes,respectively. It is cells and haploid gametes,respectively. It is during these division stages that the genetic during these division stages that the genetic material is condensed into discrete,visible material is condensed into discrete,visible structures called chromosomes.structures called chromosomes.

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CChapter contenthapter content 1.cell structure1.cell structure 2.homologous chromosome, haploidy,diploidy2.homologous chromosome, haploidy,diploidy 3.mitosis and meiosis3.mitosis and meiosis 4.cell cycle control4.cell cycle control 5.meiosis and reproduction5.meiosis and reproduction 6.spermatogenesis and oogenesis6.spermatogenesis and oogenesis 7.The significance of meiosis7.The significance of meiosis 8.The relationship between chromatin and chro8.The relationship between chromatin and chro

mosomes mosomes

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Vocabulary Terms

• Chromatin• nucleolus• NORs (nucleolar organizer reg

ions• centromere• kinetochore• Homologous chromosomes

• Locus• karyokinesis• cytokinesis• cell cycle (all phases)• sister chromatids• microtubules• molecular motors• apoptosis

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1.cell structure

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nucleoli: nuclear organucleoli: nuclear organelles containing rRNnelles containing rRNAA

nucleolar organizers: nucleolar organizers: genes encoding rRNgenes encoding rRNAA

The nucleolus

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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.

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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 2000-fold, 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

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CHROMOSOME STRUCTURE

1: nucleosomes, 10 nm = DNA + histone beadsoctomer (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

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染色体的结构

一、一、染色质的基本结构染色质的基本结构染色质（染色质（ chromatin)—chromatin)— 染色体在细胞分裂染色体在细胞分裂

间期所表现的形态间期所表现的形态 ,, 呈纤细的丝状结构，呈纤细的丝状结构，也称为染色质线也称为染色质线。。

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原核生物——裸露的环状双链原核生物——裸露的环状双链 DNADNA ，通常只有，通常只有一个。一个。 20002000 年测定发现霍乱菌（年测定发现霍乱菌（ Vibrio cholerVibrio cholerace)ace) 有两个环：有两个环： 大的——大的—— 2961kb2961kb 小的——小的—— 1072kb1072kb

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染色体的组成成分 染色体染色体 =DNA+=DNA+ 组蛋白—组蛋白— DNA.DNA. 蛋白质纤丝重复折蛋白质纤丝重复折

叠而成叠而成 DNA 1DNA 1 染色体 蛋白质染色体 蛋白质 == 组蛋白 组蛋白 1 1 （（ 55 种） 种） ++ 非组蛋非组蛋

白白 少量少量 RNARNA Kernberg(1974)Kernberg(1974) 提出染色体绳珠模型：提出染色体绳珠模型：DNA.DNA. 蛋白质纤丝基本结构单位——核小体蛋白质纤丝基本结构单位——核小体 核心颗粒核心颗粒 88 个组蛋白分子（个组蛋白分子（ H2AH2A 、、 H2BH2B 、、 H3H3 、、 H4)—140bp + H1-6H4)—140bp + H1-6

0bp0bp

Nuclesome NDANuclesome NDA 双链环绕核心外双链环绕核心外 1.3/4,140bp1.3/4,140bp

（（ 200bp 11nm) 200bp 11nm) 连接区连接区 50-6050-60 个碱基个碱基 ++ 组蛋白组蛋白此时染色体长度压缩了此时染色体长度压缩了 77 倍（ 倍（ 一级结构一级结构））

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CHROMOSOME STRUCTURE

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CHROMOSOME STRUCTURE

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一级结构一级结构

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CHROMOSOME STRUCTURE

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真核生物染色体真核生物染色体11 、常染色质：染色体的主要成分。染色较、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。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。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一部分基因进行转录

22 、异染色质、异染色质 折叠非常致密、染色较深。分裂周期中致密程度折叠非常致密、染色较深。分裂周期中致密程度

很少改变。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核膜和核仁四周。位于异染色质中的基因没有转录活性。录活性。

根据异染色质根据异染色质 DNADNA 序列的不同又可分为序列的不同又可分为 （（ 11 ）组成型异染色质 一种永久性的在染色体有）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，由不表达的固定位置的异染色质，着丝粒周围，由不表达的DNADNA 序列（卫星序列（卫星 DNADNA ）组成，稳定染色体结构）组成，稳定染色体结构的作用的作用

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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。如果蝇的有组成整条染色体。如果蝇的 YY 染色体。某些染色体。某些动物除正常染色体（动物除正常染色体（ AA 染色体）之外，还有一染色体）之外，还有一些数目不等的额外染色体（些数目不等的额外染色体（ BB 染色体），称为染色体），称为超染色体，多为异染色质构成。超染色体，多为异染色质构成。

（（ 22 ）功能型异染色质）功能型异染色质 又又 XX 异染色质，特定条件下由常染色质转变而异染色质，特定条件下由常染色质转变而

来。例如哺乳动物的来。例如哺乳动物的 XX 染色体。雌性个体细胞染色体。雌性个体细胞中有两个中有两个 XX 染色体，其中一条随机失活，处于染色体，其中一条随机失活，处于异染色质状态，而另一条有活性的异染色质状态，而另一条有活性的 XX 染色体仍染色体仍是常染色质。是常染色质。

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染色体的结构模型 KernbergKernberg 把核小体—核小体通过连接区（把核小体—核小体通过连接区（ 60bp)60bp)

以一定间隔连接形成一串珠子，称为绳珠模型。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研究得知，所谓的绳珠模型是在制备染色体时 DNDN

AA 上的上的 H1H1 丢失（或被破坏）的结果。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 ，由不同在活体细胞中，核小体与核小体贴近的 ，由不同

H1H1 相互作用，核小体卷曲盘旋呈中空状的螺线管相互作用，核小体卷曲盘旋呈中空状的螺线管——外径——外径 30nm30nm 、内径、内径 10nm10nm ，相邻的螺旋间距，相邻的螺旋间距 1111nmnm 、每一周螺旋、每一周螺旋 66 个核小体，此时又压缩了个核小体，此时又压缩了 66 倍。倍。

螺线管进一步螺旋化——超螺线管，直径螺线管进一步螺旋化——超螺线管，直径 40nm40nm 的的圆筒状结构圆筒状结构 -- 超螺旋体此时又压缩超螺旋体此时又压缩 4040 倍。倍。

超螺线管进一步螺旋或折叠——形成有丝分裂所见超螺线管进一步螺旋或折叠——形成有丝分裂所见到的染色体，此时又压缩到的染色体，此时又压缩 55 倍。倍。

84008400 倍倍 =7 X 6 X 40 X 5=7 X 6 X 40 X 5 倍。倍。

二级结构二级结构

三级结构三级结构

四级结构四级结构

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染色质环的结构

核基质(Nuclear Matri

x)

300nm

30nm 纤维(30nm fiber)

The Organization of 30nm fiber into chromosomal loops

三级结构三级结构

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CHROMOSOME STRUCTURE

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2.homologous chromosome, haploidy,diploidy

The number and shape of chromosomeThe number and shape of chromosomes vary from species to species s vary from species to species

levels of organization: levels of organization: 1. ploidy – chromosome “sets”1. ploidy – chromosome “sets”2.2.  nn – how many of each “type” – how many of each “type”3.3.  size – somewhat arbitrarysize – somewhat arbitrary4.4.  position of centromere, arm lengthposition of centromere, arm length5. landmarks – chromomeres5. landmarks – chromomeres

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。

                                                                                                                

                                             

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CHROMOSOME karyotpye

human chromosome set: 23 pairshuman chromosome set: 23 pairs GroupingGrouping Banding Banding

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2. 4 .Chromosome classification and karyotypeChromosomes are identifiable based on the position of the centromere

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Chromosome classification

Type long/short arm symbol division behavior

中间着丝粒 1-1.7 M.m V metacentric chro

近中着丝粒 1.7-3 Sm L submetacentric chro

近端着丝粒 3-7 St l acrocentric chro

顶端着丝粒 7— T.t l acrocentric chro

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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

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G-banding - Giemsa stainR-banding - reverse bandingC-bandingQ-bandingFluorescent banding

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Banded chromosomeBanded chromosome (give differential stai(give differential staining along the length of chromosome)ning along the length of chromosome)

Intercalating agentIntercalating agent: : staining compound thstaining compound that insert between the base-pairs of DNAat insert between the base-pairs of DNA

QuinacrineQuinacrine: a fluorescent compound (are de: a fluorescent compound (are detected only when DNA are exposed to Ultravitected only when DNA are exposed to Ultraviolet). Quinacrine that have inserted into the olet). Quinacrine that have inserted into the chromosome to emit energy. Parts of the chrchromosome to emit energy. Parts of the chromosome shine brightly, whereas other partomosome shine brightly, whereas other parts remain dark.s remain dark.

The Staining procedure is called: The Staining procedure is called: Q-bandingQ-banding The Bands that it produces are called: The Bands that it produces are called: Q-banQ-bandd

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The banding patterns depend on staining procedures andthe extent of chromosomal condensation.

1.Q-banding: Stain with Quinacrine or similar fluorescentdye view by UV fluorescence

Types of staining:

2. G-banding: Pretreat with trypsin, stain with Giemsa3. R-banding: Heat-denature then stain with Giemsa.

4. C-banding: Denature with saturated Ba(OH)2 thenstain with Giemsa.

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Idiogram

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Morphological characteristics of chromosomes - Size

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CHROMOSOME TOPOGRAPHY

Drosophila Drosophila chromosomeschromosomes centromerescentromeres telomeres (later)telomeres (later) euchromatineuchromatin heterochromatinheterochromatin polytene chromosomespolytene chromosomes

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CHROMOSOME TOPOGRAPHY

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• Review the process of mitosis, and observe the 4 phases of mitosis

•Review the process of meiosis, and observe the various phases of meiosis

3.mitosis and meiosis

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Cell division and Cell Cycle: the events that occur Cell division and Cell Cycle: the events that occur from the completion of one round of division to from the completion of one round of division to the beginning of the nextthe 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 (M-

phase entry)

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Most of the Cell Cycle is spent in interphase

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4.Cell Cycle regulation (control) The cell cycle ,including both mitosis and miosis ,is fuThe cell cycle ,including both mitosis and miosis ,is fu

ndamentally the same in all eukaryotic organisms. ndamentally the same in all eukaryotic organisms. The similarity of the event leading to cell duplication iThe similarity of the event leading to cell duplication i

n various organisms indicated that :n various organisms indicated that : Governed by genetic program Governed by genetic program Conserved throughout evolution .Conserved throughout evolution . Disruption of this regulation may lead to the uncontrollDisruption of this regulation may lead to the uncontroll

ed cell division , which is related with the cancer.ed cell division , which is related with the cancer. Many genes control the cell cycleMany genes control the cell cycle Mutations Mutations cdc(cell division cycle)thatcdc(cell division cycle)that interrupt the cell interrupt the cell

cycle are funded cycle are funded The study of these mutations has established at least thrThe study of these mutations has established at least thr

ee major checkpoints exist.ee major checkpoints exist.

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Regulation of the Eukaryotic cell cycle Two critical events:Two critical events: 1. Nuclear DNA replication1. Nuclear DNA replication 2. Mitosis (cell division)2. Mitosis (cell division) are fundamentally similar in all eukaryotic celare fundamentally similar in all eukaryotic cel

lsls Master controller of these events ---Master controller of these events --- heterodimeric protein kinaseheterodimeric protein kinase

regulatory subunits catalytic subunit

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Postmitotic cells:

“exit” the cell cycle

G0

Restriction pointthe point in late G1where passage throughthe cell cycle becomeindependent of mitogens

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•Cyclins and Cdk Cyclin-dependent kinaseCyclin-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

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Cell cycle

A variety of genes and proteins cotroll the eA variety of genes and proteins cotroll the event of cell cycle. These genes and proteins vent of cell cycle. These genes and proteins allow progression to next stage of the cycleallow progression to next stage of the cycles when all is well, but cause to brake when s when all is well, but cause to brake when damage to the genome.damage to the genome.

Cyclin-dependent kinaseCyclin-dependent kinase collaborate with collaborate with ccyclinyclin to ensure proper time and sequence of to ensure proper time and sequence of cell cycle events.cell cycle events.

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70 cell cycle genes are found CDKsCDKsCDC28 from CDC28 from saccharomyces cerevisiaesaccharomyces cerevisiaeCDC 2 CDC 2 schizsaccharomyces cerevisiaeschizsaccharomyces cerevisiaeCDK4CDK4CDK2CDK2 CYCLINSCYCLINSDDEEAABB

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G1 to S phase

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G2 to M phase

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Mitosis yields two identical diploid cells

Meiosis yields four daughter cells that are not identical

5.Meiosis and sexual

reproduction

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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)

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•Metapase I- chiasma still evident, homologues align randomly across equator

•Anaphase I- homologues separate

•Telophase I- variable, from none to short interphase

•Note that cells are now haploid

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2nd Meiotic Division ensures each daughter cell receives a single chromatid from original tetrad

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6.Spermatogenesis and Oogenesis

Oogenesis occurs in the ovaries and produces one haploid egg

Spermatogenesis occurs in the testis and produces 4 haploid sperm

•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

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动物生活史

初级卵母细胞(Primary oocyte)

卵子形成(Oogenesis)

精子形成(Spermatogenesis)

初级精母细胞(Primary spermatocyte)

次级精母细胞(Secondary spermatocyt

e)精细胞

（ Spermatids ）

精子（ spermatozoa)

第一极体(First polar body)

卵细胞(ovum)

第二极体(Second polar body)

性原细胞（ gonia)

次级母细胞

初级母细胞

次级卵母细胞（ Secondary oocyte)

配子(gametes)

合子(zygote)

I

II

卵原细胞(oogonia)

精原细胞(spermatogonia)

受精卵(oosperm) 胚胎

(embryo)动物个体

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Multicellular plants alternate between haploid and diploid generations

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Plant life cycle

雄花

Microspore mother cell 小孢子母细胞 (2n)

Microspore 小孢子

MMC 大孢子母细胞 (2n)

大孢子 (n)

meiosismeiosis

Pollen花粉

Mature Pollen 成熟花粉

精核 (n)发芽花粉

胚囊 (n)

极核 卵核

雌花

种子胚乳 (3n)胚 (2n)

Microsporogenesis

A mature pollen grain has three (N) nuclei [2 sperm nuclei + 1 Tube nucleus].

The (N) embryo sac originally had 8 (N) nuclei but two fused together so the functional mature embryo sac (female gametophyte) has 7.

Megasporogenesis

Endosperm (3N)

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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.

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一个雄核 +卵核——二倍体 2n——胚

一个雄核 +极核——三倍体 3n——胚乳Double Fertilization

1 Sperm Nucleus (N) + 1 Egg Nucleus

(N)

=

Zygote (2N)

Mitosis

Embryo (2N)

Mitosis

Mature Adult Plant (2N)

1 Sperm Nucleus (N)+

1 Fused Polar Nucleus (2N)

=Endosperm (3N)

[lining of endosperm is aleurone,

which is also 3N]

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Functional Significance of Meiosis is Increased Genetic Variability While Ensuring

Genetic Constancy Between Generations

7.The significance of Meiosis

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Sexual Sources of Genetic Variation For asexually reproducing organisms, the only For asexually reproducing organisms, the only

source of genetic variation is source of genetic variation is mutationsmutations.. For sexually reproducing organisms, there are For sexually reproducing organisms, there are

three additional mechanisms for generating three additional mechanisms for generating genetic variation:genetic variation:1. Independent assortment of chromosomes.1. Independent assortment of chromosomes.

2n Combinations of Maternal and paternal Chromosomes

2. Crossing Over3. Random fertilization.3. Random fertilization.

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Independent Assortment of Chromosomes Homologous chromosome pairs are Homologous chromosome pairs are

oriented at random along the metaphase oriented at random along the metaphase plate during meiosis I.plate during meiosis I.

Each chromosome of a homologous pair Each chromosome of a homologous pair has an equal probability of ending up in one has an equal probability of ending up in one or the other daughter cell.or the other daughter cell.

For N homologous pairs of chromosomes, For N homologous pairs of chromosomes, there are 2there are 2NN possible types of gametes. possible types of gametes.

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For examples:

For For Drosophila melanogasterDrosophila melanogaster, there are 4 h, there are 4 homologous pairs of chromosomes (2N = 8); omologous pairs of chromosomes (2N = 8); therefore, there are 16 possible types of gatherefore, there are 16 possible types of gametes (2metes (244).).

For For Homo sapiensHomo sapiens (humans), there are 23 h (humans), there are 23 homologous pairs of chromosomes (2N = 46);omologous pairs of chromosomes (2N = 46); therefore, there are 8,388,608 possible type therefore, there are 8,388,608 possible types of gametes (2s of gametes (22323).).

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Independent Assortment of Chromosomes - continued

Type 1 Type 2 Type 3 Type 4

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Random Fertilization

During fertilization, haploid gametes from During fertilization, haploid gametes from two genetically different individuals are two genetically different individuals are combined to form a diploid zygote.combined to form a diploid zygote.

If each parent can produce a total of X If each parent can produce a total of X different types of gametes, then the total different types of gametes, then the total possible number of genetically unique possible number of genetically unique zygotes is Xzygotes is X22..

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For example

, ignoring crossing over, , ignoring crossing over, Drosophila melanDrosophila melanogaster ogaster can produce 2can produce 244 = 16 different gamet = 16 different gametes through the independent assortment of ches through the independent assortment of chromosomes. Therefore, each male and femaromosomes. Therefore, each male and female fly can produce 2le fly can produce 244 x 2 x 244 = 2 = 288 = 256 genetic = 256 genetically unique combinations of zygotes.ally unique combinations of zygotes.

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For example

ignoring crossing over, humans can ignoring crossing over, humans can produce 2produce 22323 = 8,388,608 different gametes = 8,388,608 different gametes through the independent assortment of through the independent assortment of chromosomes. Therefore, each male and chromosomes. Therefore, each male and female human can produce 2female human can produce 22323 x 2 x 22323 = 2 = 24646 = = 70,368,744,177,664 genetically unique 70,368,744,177,664 genetically unique combinations of zygotes.combinations of zygotes.

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8.The relationship between Chromatin and Chromosome Each chromosome consists of oneEach chromosome consists of one

long strand of DNA, along with itslong strand of DNA, along with its

associated proteins. This complex ofassociated proteins. This complex of

DNA and protein is calledDNA and protein is called

““chromatinchromatin”.”.

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Page 80: 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

EndEnd


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