Cell Reproduction Chapter 19

Understanding Cell Division What instructions are necessary for inheritance? How are those instructions duplicated for distribution into daughter cells? By what mechanisms are instructions parceled out to daughter cells?

Reproduction Parents produce a new generation of cells or multicelled individuals like themselves Parents must provide daughter cells with hereditary instructions, encoded in DNA, and enough metabolic machinery to start up their own operation

Eukaryotic Cell Division Mitosis, division of cytoplasm Body growth and tissue repair Meiosis, division of cytoplasm Formation of gametes, sexual reproduction

Chromosome A DNA molecule & attached proteins Duplicated in preparation for mitosis one chromosome (unduplicated) one chromosome (duplicated)

Chromosome Number 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

Cell Cycle Cycle starts when a new cell forms During cycle, cell increases in mass and duplicates its chromosomes Cycle ends when the new cell divides

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

HeLa Cells Line of human cancer cells that can be grown in culture Descendants of tumor cells from a woman named Henrietta Lacks Lacks died at 31, but her cells continue to live and divide in labs around the world

Mitosis Period of nuclear division Usually followed by cytoplasmic division Four stages: Prophase Metaphase Anaphase Telophase

Stages of Mitosis Prophase 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 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 in unduplicated form

Cytoplasmic Division Usually occurs between late anaphase and end of telophase

Effects of Irradiation Ionizing radiation May break apart chromosomes, alter genes Large doses destroy cells Small doses over long time less damaging Medical X rays, radiation therapy

Irradiated Food Foods irradiated to kill microorganisms, prolong shelf life Not radioactive No evidence it is a health risk Critics worry about selecting for radiation-resistant strains, changes in food composition

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

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

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 Moves chromosomes during mitosis

Sexual Reproduction Chromosomes are duplicated in germ cells Germ cells undergo meiosis and cytoplasmic division Cellular descendants of germ cells become gametes Gametes meet at fertilization

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

Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

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)

Spermatogenesis secondary spermatocytes (haploid) spermato- gonium (diploid male- reproductive cell) primary spermatocyte (diploid) spermatids (haploid) Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division Growth

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

Stages of Meiosis Meiosis I Meiosis II Prophase II Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II

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 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

Random Alignment 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

Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which 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 & Meiosis Compared Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring

Prophase vs. Prophase I Prophase (Mitosis) Prophase I (Meiosis) Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs

Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) Homologous chromosomes are separated from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome are separated from each other

Results of Mitosis and Meiosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another