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 – Interval or gap after cell division

• S– 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 and proteinsarranged as cylindrical fiber

DNA

histone

one nucleosome

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

testes

ovaries

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

one chromosome (duplicated)

two chromosomes (unduplicated)

Spermatogenesis

GrowthMitosis I,

Cytoplasmic divisionMeiosis II,

Cytoplasmic division

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

spermato-gonium

(diploid male- reproductive

cell)

Oogenesis

Growth Mitosis I,Cytoplasmic division

Meiosis II,Cytoplasmic division

ovum (haploid)

primary oocyte (diploid)

oogonium (diploid

reproductive cell) secondary

oocyte (haploid)

first polar body

(haploid)

three polar bodies

(haploid)

Stages of Meiosis

Meiosis I

• 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

or

or

or

1 2 3

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• 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

Prophase vs. Prophase I

• Prophase (Mitosis)

– 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

• Mitosis

– Two diploid cells produced

– Each identical to parent

• Meiosis

– Four haploid cells produced

– Differ from parent and one another