第五章 Chromosomes: Where are the Genes Found? 前言

chromosome (unduplicated) in a cell at interphase same chromosome (duplicated) in interphase prior to mitosis mitosis, cytoplasmic division Fig. 8.6, p. 131 chromosome (unduplicated) in a daughter cell at interphase chromosome (unduplicated) in a daughter cell at interphase

分裂的細胞: 世代間的橋樑 分裂機制的概觀 有關染色體的要點 有絲分裂與染色體的數目

細胞的分裂 有絲分裂 (Mitosis) vs. 減數分裂 (Meiosis) 體細胞 (Somatic cell) vs. 生殖細胞 (Germ cell)

Prokaryotic organisms Division Mechanisms Eukaryotic organisms Mitosis Meiosis Prokaryotic organisms Prokaryotic fission

Roles of Mitosis Multicelled organisms Growth Cell replacement Some protistans, fungi, plants, animals Asexual reproduction

細看染色體 姊妹染色分體 著絲粒

Organization of Chromosomes DNA one nucleosome DNA and proteins arranged as cylindrical fiber histone

One nucleosome DNA core of histone Fig. 8.3, p. 129

單套 (Haploid) vs. 雙套 (Diploid) 有絲分裂與染色體的數目 單套 (Haploid) vs. 雙套 (Diploid) Sum total of chromosomes in a cell Somatic cells Chromosome number is diploid (2n) Two of each type of chromosome Gametes Chromosome number is haploid (n) One of each chromosome type

Human Chromosome Number Diploid chromosome number (n) = 46 Two sets of 23 chromosomes each One set from father One set from mother Mitosis produces cells with 46 chromosomes--two of each type

Lots of DNA Stretched out, the DNA from one human somatic cell would be more than two meters long A single line of DNA from a salamander cell would extend for ten meters

細胞週期 (Cell Cycle)

Spindle Apparatus one spindle pole one of the condensed chromosomes spindle equator microtubules organized as a spindle apparatus one spindle pole

Interphase Usually longest part of the cycle Cell increases in mass Number of cytoplasmic components doubles DNA is duplicated

Stages of Interphase G1 S G2 Interval or gap after cell division Time of DNA synthesis (replication) G2 Interval or gap after DNA replication

Control of the Cycle Once S begins, the cycle automatically runs through G2 and mitosis The cycle has a built-in molecular brake in G1 Cancer involves a loss of control over the cycle, malfunction of the “brakes”

Stopping the Cycle Some cells normally stop in interphase Neurons in human brain Arrested cells do not divide Adverse conditions can stop cycle Nutrient-deprived amoebas get stuck in interphase

有絲分裂之階段 前期 (Prophase) 中期 (Metaphase) 後期 (Anaphase) 末期 (Telophase)

前期:有絲分裂之開始

過度到中期 後期到末期

胞質分裂 (Cytokinesis; Cytoplasmic division) 植物有細胞板的形成 動物細胞的胞質分裂

植物: 細胞板 (Cell plate) 的形成

動物細胞的胞質分裂 分割溝 (Cleavage furrow)

Culturing Cells Growing cells in culture allows researchers to investigate processes and test treatments without danger to patients Most cells cannot be grown in culture

減數分裂 前言: 托鞋笠貝(Slipper limpet)的故事 芽蟲 (Aphids)的故事

托鞋笠貝 (Slipper limpet) 芽蟲 (Aphids)

Sexual Reproduction 有性生殖與無性生殖的比較 Chromosomes are duplicated in germ cells Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Gametes meet at fertilization

Asexual Reproduction 有性生殖與無性生殖的比較 Single parent produces offspring All offspring are genetically identical to one another and to parent

Sexual Reproduction Involves Meiosis Gamete production Fertilization Produces genetic variation among offspring

Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells

Chromosome Number 減數分裂(Meiosis)使染色體數目 (Chromosome number)減半 Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

物種 雙套染色體數 果蠅 8 豌豆 14 玉米 20 青蛙 26 蚯蚓 36 人類 46 猩猩 48 阿米巴變形蟲 50 木賊 216

減數分裂是兩次分裂,不是一次 Two consecutive nuclear divisions Meiosis I Meiosis II DNA is NOT duplicated between divisions Four haploid nuclei are formed

Meiosis I Each homologue in the cell pairs with its partner, then the partners separate

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated)

減數分裂各時期

減數分裂各時期

減數分裂的重要事件 前期 中期 (排列)

前期

中期 (排列)

從配子到子代 植物的配子形成 動物的配子形成

植物的配子形成

動物的配子形成: 精子形成(Spermatogenesis)與卵子形成(Oogenesis)

精子形成(Spermatogenesis)

卵子形成(Oogenesis)

減數分裂與有絲分裂之比較

減數分裂與有絲分裂之比較

減數分裂與有絲分裂之比較

授精作用(Fertilization) 恢復染色體數目 有更多的遺傳特性之交換

Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Non-sister chromosomes exchange segments

Effect of Crossing Over After crossing over, each chromosome contains both maternal and parental segments Creates new allele combinations in offspring

Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents

Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)

Possible Chromosome Combinations 1 2 3 Possible Chromosome Combinations or or or