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 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 Locus Chromatin karyokinesis nucleolus cytokinesis cell cycle (all phases) sister chromatids microtubules molecular motors apoptosis Chromatin nucleolus NORs (nucleolar organizer regions centromere kinetochore Homologous chromosomes

1.cell structure

nucleoli: nuclear organelles containing rRNA 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..

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

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 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 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）

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。

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

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 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——形成有丝分裂所见到的染色体，此时又压缩5倍。 8400倍=7 X 6 X 40 X 5倍。 二级结构 三级结构 四级结构

染色质环的结构 三级结构 30nm纤维 (30nm fiber) 300nm 核基质 (Nuclear Matrix) The Organization of 30nm fiber into chromosomal loops

CHROMOSOME STRUCTURE

2.homologous chromosome, haploidy,diploidy 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

。                                                                                                                                                              

CHROMOSOME karyotpye 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 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

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

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

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 when to initiate replication (S-phase entry) when to begin chromosomal condensations (M-phase 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.

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 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.

70 cell cycle genes are found CDKs CDC28 from saccharomyces cerevisiae CDC 2 schizsaccharomyces cerevisiae CDK4 CDK2 CYCLINS D E A B

G1 to S phase

G2 to M phase

5.Meiosis and sexual reproduction 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 I- homologues 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

6.Spermatogenesis and Oogenesis 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

(Primary spermatocyte) (Secondary spermatocyte) 动物生活史 精子形成 (Spermatogenesis) 卵子形成 (Oogenesis) 性原细胞 （gonia) 精原细胞 (spermatogonia) 卵原细胞 (oogonia) 初级精母细胞 (Primary spermatocyte) 初级母细胞 初级卵母细胞 (Primary oocyte) I 次级精母细胞 (Secondary spermatocyte) 次级母细胞 次级卵母细胞 （Secondary oocyte) 第一极体 (First polar body) II 精细胞 （Spermatids） 配子 (gametes) 第二极体 (Second polar body) 卵细胞 (ovum) 精子 （spermatozoa) 合子 (zygote) 受精卵 (oosperm) 胚胎 (embryo) 动物个体

Multicellular plants alternate between haploid and diploid generations

Plant life cycle Microsporogenesis MMC大孢子母细胞(2n) Megasporogenesis 雄花 MMC大孢子母细胞(2n) 雌花 Microspore mother cell 小孢子母细胞(2n) meiosis meiosis Endosperm (3N) 大孢子(n) Microspore小孢子 种子 胚乳(3n) 胚(2n) Pollen花粉 极核 卵核 Mature Pollen成熟花粉 The (N) embryo sac originally had 8 (N) nuclei but two fused together so the functional mature embryo sac (female gametophyte) has 7. 胚囊(n) A mature pollen grain has three (N) nuclei [2 sperm nuclei + 1 Tube nucleus]. 精核(n) 发芽花粉

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.

Double Fertilization 1 Sperm Nucleus (N) + 1 Egg Nucleus (N) = Zygote (2N) Mitosis Embryo (2N) Mature Adult Plant (2N) 1 Sperm Nucleus (N) + 1 Fused Polar Nucleus (2N) = Endosperm (3N) [lining of endosperm is aleurone, which is also 3N]

7.The significance of Meiosis Functional Significance of Meiosis is Increased Genetic Variability While Ensuring Genetic Constancy Between Generations

Sexual Sources of Genetic Variation 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.

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).

Independent Assortment of Chromosomes - continued Type 1 Type 2 Type 4 Type 3

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.

For example , 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 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”.

End