How Cells Reproduce

Dividing HeLa Cells

Eukaryotes and Prokaryotes

Eukaryotic cells undergo mitosis and/or meiosis• Separates duplicated chromosomes of parent cell

into two daughter nuclei• Another mechanism divides cytoplasm

Mitosis and Meiosis

Mitosis• Basis of growth, cell replacements, and tissue

repair in multicelled species• Basis of asexual reproduction in many single-

celled and multicelled species

Meiosis• Basis of sexual reproduction• Precedes formation of gametes or sexual spores

Cell Division Mechanisms

Chromosome Structure

Eukaryotic chromosome • Association of DNA, histones, and other proteins

Nucleosome • Smallest unit of organization• Double-stranded DNA looped twice around a

spool of histones

Structure of a Condensed Chromosome

Sister Chromatids

A duplicated chromosome consists of two sister chromatids

Sister chromatids remain attached at their centromere until late in mitosis

Introducing the Cell Cycle

Cell cycle • Starts when a new cell forms • Runs through interphase• Ends when cell reproduces by nuclear and

cytoplasmic division

Interphase

Most cellular activities occur in interphase• G1: Cell grows in mass, doubles number of

cytoplasmic components• S: DNA replication duplicates chromosomes• G2: Cell prepares for division

Fig. 8.5, p.128

Interval of cell growth beforeDNA replication (chromosomesunduplicated)

G1

cytoplasmicdivision; eachdaughter cellenters interphase

Interphase ends for parent cell

Interval after DNAreplication; the cellprepares to divide

G2

Interval of cell growth whenthe DNA is replicated (allchromosomes duplicated)

S

Chromosome Number

Sum of all chromosomes in cells of a given type

In human body cells, chromosome number is 46

Body cells are diploid (have two of each kind of chromosome)

Mitosis and Chromosome Number

Mitosis maintains parental chromosome number from one generation to the next• Bipolar spindle divides sister chromatids

A Closer Look at Mitosis

Mitosis • A nuclear division mechanism that maintains the

chromosome number

Mitosis proceeds in four stages:• Prophase• Metaphase• Anaphase• Telophase

Prophase

Duplicated chromosomes become threadlike as they start to condense

Microtubules form a bipolar spindle

Nuclear envelope starts to break apart

Metaphase

All chromosomes become aligned midway between the two spindle poles• Chromosomes in most condensed forms

Anaphase

Sister chromatids detach from each other• Spindles move them toward opposite poles

Telophase

Chromosomes reach opposite spindle poles

Nuclear envelope forms around each cluster

Both new nuclei have the parental chromosome number

Mitosis

Mitosis

Fig. 8.7a, p.130

Fig. 8.7b, p.130

nuclearenvelope chromosome

Mitosis begins. DNA and its associatedproteins have started to condense. Twochromosomes (color-coded purple) wereinherited from the female parent. Theother two (blue) are their counterparts,inherited from the male parent.

b Early Prophase

Fig. 8.7c, p.130

pair of centrioles

The duplicated chromosomes continue to condense. New microtubules move one of two pairs of centrioles to the opposite side of the nucleus. The nuclear envelope starts to break up.

c Late Prophase

Fig. 8.7e, p.130

e MetaphaseAll of the chromosomes havebecome lined up midwaybetween the spindle poles.At this stage of mitosis, thechromosomes are in theirmost tightly condensed form.

Fig. 8.7f, p.130

Sister chromatids separate asmotor proteins moving alongspindle microtubules dragthem to opposite spindlepoles. Other microtubulespush the poles farther apart.

f Anaphase

Fig. 8.7g, p.130

There are two clusters ofchromosomes, which nowdecondense. Patches ofnew membrane fuse to forma new nuclear envelope.Mitosis is over.

g Telophase

Fig. 8.7h, p.130

After cytoplasmic division,there are two daughtercells. Each is diploid: Itsnucleus has two of eachtype of chromosome, justlike the parent cell.

h Two DaughterCells at Interphase

Cytoplasmic Division Mechanisms

Mechanisms of cytoplasmic division differ in plant and animal cells

In animal cells• A contractile ring of microfilaments (part of cell

cortex) contracts and pulls the cell surface inward until the cytoplasm is divided

Cytoplasmic Division in Animal Cells

Cytoplasmic Division in Plant Cells

Results of Cell Division: A Human Embryo

When Control is Lost

Mutant checkpoint genes can cause tumors by disrupting normal controls