How Cells Reproduce

Chapter 7

Henrietta’s Immortal Cells

• HeLa cells

• Derived from cervical cancer that killed Henrietta Lacks

• First human cells to grow and dividein culture

• Used in research throughout the world

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

Chromosome

• A DNA molecule and attached proteins

• Duplicated in preparation for cell division

one chromosome (unduplicated)

one chromosome (duplicated)

INTERPHASE

SInterval of cellgrowth, whenDNA replicationis completed(chromosomesduplicated)

Inte

rpha

se e

nds

for

pare

nt c

ell

Cytoplasmic division

G1Interval of cellgrowth, beforeDNA replication(chromosomesunduplicated)

G2Interval followingDNA replication;cell preparesto divide

Each daughter cellstarts interphase

MITO

SIS

Telophase

Anaphase

Metaphase

Prop

hase

Fig. 7-2, p.96

The Cell Cycle

The cell cycle

The Cell Cycle

Control of the Cycle

• Once S begins, the cycle usually

runs through G2 and mitosis

• Cycle has a built-in molecular brake

in G1

• Cancer involves a loss of control

over the cycle, malfunction of

“brakes”

Interphase

• Usually the longest part of cycle

• Cell increases in mass

• Number of cytoplasmic components

doubles

• DNA is duplicated

Mitosis

• Period of nuclear division

• Usually followed by cytoplasmic division

• Four stages:ProphaseMetaphaseAnaphaseTelophase

Chromosome Number

• Total number 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 – One set from father– One set from mother

• Mitosis produces cells with 46 chromosomes: two of each type

Human Chromosomes

Maintaining Chromosome

Number

chromosomes (unduplicated) in parent cell at interphase

same chromosomes (duplicated) in interphase prior to mitosis

mitosis, cytoplasmic division

chromosome (unduplicated) in daughter cell at interphase

chromosome (unduplicated) in daughter cell at interphase

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

Spindlesspindle microtubules

chromosomes at spindle equator, midway between spindle poles

Stages of Mitosis

Prophase

Metaphase

Anaphase

Telophase

Mitosis step-by-step

Mitosis

Cytoplasmic Division

• Usually occurs between late anaphase and end of telophase

• Two mechanisms

– Cleavage (animals)– Cell plate formation (plants)

animal cell pinching in two

Cytoplasmic division

Cleavage & Cell Plate Formation

Asexual Reproduction

• 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

• Chromosome pairs: one from mother,

one from father

• Paternal and maternal chromosomes

carry different alleles

Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits

• Variation in traits is the basis for evolutionary change

Gamete Formation

• Gametes are sex cells (sperm, eggs)

• Arise from germ cells in reproductive organs

testes

ovaries

Reproductive organs

Gamete Formation

Chromosome Number

• Total number of chromosomes

in cell

• Germ cells are diploid (2n)

• Gametes are haploid (n)

• Meiosis halves chromosome

number

Diploid To Haploid

one chromatid

its sisterchromatid

centromere

one chromosome in the duplicated state

each homologue in the cell pairs with its partner (synapsis) then partners separate

Stepped Art

p.103

two chromosomes (unduplicated)

one chromosome (duplicated)

Meiosis: Two Divisions

• Two consecutive nuclear divisions – Meiosis I

– Meiosis II

• DNA is not duplicated between divisions

• Four haploid nuclei form

p.102

Meiosis step-by-step

Meiosis

Sexual Reproduction and Genetic Variation

• Two functions of meiosis provide variation in traits:– crossing over– random alignment

Crossing Over

• Each chromosome attaches to its homologue

• All four chromatids are closely aligned (synapsis)

• Nonsister chromatids exchange segments

Crossing over

Crossing Over

Effects of Crossing Over

• After crossing over, each chromosome

contains both maternal and paternal

segments

• Creates new allele combinations in

offspring

Random Alignment

• Between prophase I and metaphase I, chromosome pairs align randomly at metaphase plate

• Initial contact between microtubule and either maternal or paternal chromosome is random

Random alignment

Animal Life Cycle

Factors Contributing to Variation among Offspring

• Crossing over during prophase I

• Random alignment of chromosomes at metaphase I

• Random combination of gametes at fertilization

Fertilization

• Male and female gametes unite

and nuclei fuse

• Fusion of two haploid nuclei produces

diploid nucleus in zygote

• Random chance of sperm fertilizing egg

increases variation in offspring

Sperm formation

Animal Sperm Formation

Egg formation

Animal Egg Formation

Cancer Characteristics

• Plasma membrane and cytoplasm

altered

• Cells grow and divide abnormally

• Weakened capacity for adhesion;

cells can move to new tissues

• Lethal unless eradicated

Cancer and metastasis

Cancer

Mitosis

• Functions– Asexual reproduction– Growth, repair

• Occurs insomatic cells

• Produces clones

Mitosis & Meiosis Compared

Meiosis

• Function – Sexual reproduction

• Occurs in germ cells

• Produces variable offspring

Comparing mitosis and meiosis

Results of Mitosis and Meiosis