How Cells Reproduce Chapter 7

Types of Cell Division Eukaryotic organisms –Mitosis –Meiosis Prokaryotic organisms –Prokaryotic fission

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

Chromosome A DNA molecule and attached proteins Duplicated in preparation for cell division

Organization of Chromosomes DNA wound tightly around histones before replication

The Cell Cycle

Interphase Usually longest part of the cycle (varies by cell type) G1 phase normal cell function and growth S phase DNA is replicated G2 phase proteins made for division

Prophase Metaphase Anaphase Telophase PMAT Mitosis Period of nuclear division Usually followed by cytoplasmic division

Control of the Cycle Once S begins, the cycle usually 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”

Chromosome Number Sum total of chromosomes in a cell Somatic cells –Chromosome number is diploid (2n) –Two of each type of chromosome –One from mom and one from dad Gametes –Chromosome number is haploid (n) –One of each chromosome type Why are haploid cells needed for gametes?

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 (diploid) chromosomes: two of each type

The Spindle Apparatus Consists of two distinct sets of microtubules –Each set extends from one of the cell poles –Two sets overlap at spindle equator –Begins in centrosome (has centrioles in animals) Moves chromosomes during mitosis

Maintaining Chromosome Number chromosomes (unduplicated) in parent cell at interphase

Stages of Mitosis Prophase (early, late) Metaphase Anaphase Telophase

Early Prophase: Mitosis Begins Duplicated chromosomes begin to condense

Late Prophase New microtubules are assembled One centriole pair is moved toward opposite pole of spindle Nuclear envelope starts to break up

Transition to Metaphase Spindle forms Spindle microtubules become attached to the kinetochore on the two sister chromatids of each chromosome

Metaphase All chromosomes are lined up at the spindle equator Chromosomes are maximally condensed

Anaphase Sister chromatids of each chromosome are pulled apart Once separated, each chromatid is a chromosome

Telophase Chromosomes decondense Two nuclear membranes form, one around each set of unduplicated chromosomes

Results of Mitosis Two daughter nuclei Each with same chromosome number as parent cell Chromosomes are in unduplicated form

Cytoplasmic Division Usually occurs between late anaphase and end of telophase Two mechanisms –Cell plate formation (plants) –Cleavage (animals)

Cell Plate Formation

Animal Cell Division Actin and myosin filaments pull in membrane and pinch off

Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent Parthenogenesis

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

Chromosome From Dad Gene for eye color Genes and alleles Chromosome From Mom An allele is a different expression of the same gene

Karyotype

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits This variation in traits is the basis for evolutionary change “Better” traits should lead to more offspring which over time will change the gene pool

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells in reproductive organs testes ovaries

Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) (both sets of chromosomes) Gametes are haploid (n) (only one set of chromosomes) Meiosis halves chromosome number

Meiosis: Two Divisions Two consecutive nuclear divisions –Meiosis I –Meiosis II DNA is not duplicated between divisions Four haploid nuclei form from one diploid cell

Prophase I Each duplicated chromosome pairs with homologue forming a tetrad Crossing over -- Homologues swap segments Each chromosome becomes attached to spindle

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

Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal 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

Possible Chromosome Combinations

Y chromosomes rarely cross over with X You can trace a lineage of males quite readily East Asian men: 1.5 million descendents today 16 million descendents today Giocangga Genghis Khan

Metaphase I Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed

Anaphase I Homologous chromosomes segregate The sister chromatids remain attached

Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division Are these cells haploid or diploid?

Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes

Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles

Anaphase II Sister chromatids separate to become independent chromosomes

Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells, genetically different

Spermatogenesis Growth Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division cell differentiation, sperm formation spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid ) sperm (mature, haploid male gametes) Spermatogenesis

Develop from the spermatogonium germ cell (2n) Meiosis happens as they migrate through the testes Produces four viable sperm

Oogenesis Growth Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division ovum (haploid) primary oocyte (diploid) oogonium (diploid) secondary oocyte (haploid) first polar body (haploid) three polar bodies (haploid)

Oogenesis Oogonium give rise to primary oocyte during embryonic development Primary oocyte remains in prophase I until activated (one at a time) during puberty by hormones Hormones start meiosis until metaphase II Meiosis only complete when contact with sperm is made

Oogenesis When primary oocyte divides one secondary oocyte receives most of the cytoplasm Similarly, when the secondary oocyte divides the ovum gets most cytoplasm Polar bodies degenerate

Oogenesis vs spermatogenesis 1. One gamete (egg) vs four gametes (sperm) 2. At birth female has a set number of primary oocytes 3. Oogenesis is interrupted with resting stages while spermatogenesis is continuous

Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which of the two gametes unite is random –Adds to variation among offspring

Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

Mitosis Functions –Asexual reproduction –Growth, repair Occurs in somatic cells Produces clones Mitosis & Meiosis Compared Meiosis Function –Sexual reproduction Occurs in germ cells Produces variable offspring

Animal Life Cycle Usually pretty rapid from meiosis to fertilization

Plant Life Cycle Not as rapid, from spores Spores not a fully developed gamete Can last for many years