The Cell Cycle
The Cell Theory: All organisms consist of cells and
arise from preexisting cells

Mitosis is the process by which new cells are generated.

Meiosis is the process by which gametes are generated
for sexual reproduction.
Cell and Chromosome Structure

Prokaryotic cells have no nucleus

DNA is organized into one circular chromosome
Ribosomes
Flagella
Cytoplasm
Chromosome
Chromosome
Cell wall Cell membrane
Cell and Chromosome Structure

In eukaryotic cells, the genetic material is found within a
membrane-bound nucleus

DNA is organized into multiple linear shaped chromosomes
Chromosome
Nucleus
Nuclear
membrane
Cell membrane
Cytoplasm
DNA is packaged
into chromosomes
Eukaryotic chromosomes may be in their
unreplicated or unreplicated state
Sister chromatids
Centromere
DNA replication
One chromosome
(Unreplicated state)=
One-double stranded DNA
molecule
One chromosome
(Replicated state)=
Two double stranded DNA
molecules
Chromosomes in cells

All individuals of a particular species have a characteristic number of
chromosomes
• Fruit flies have 8 chromosomes in their nuclei
• Humans have 46 chromosomes in their nuclei

Chromosomes of most species come in pairs
One pair of homologous
chromosomesUnreplicated state
One pair of homologous
chromosomesReplicated state
Homologous chromosomes

Each member of a pair of homologous chromosomes:
• Same size
• Same shape, centromere location
• Same genetic loci = physical location of a gene on a chromosome
Gene for eye color
Gene for eye color
One pair of homologous chromosomes
Homologous chromosomes

But:
• Each member of the pair (each “homolog”) may carry different
versions of the gene
• Different versions of the same gene = alleles
• Eye color gene:
 Brown allele
 Blue allele
Brown eye allele
Blue eye allele
A cell with 2 pairs of homologous chromosomes
B
Eye color gene:
B= brown eye allele
b= blue eye allele
Height gene:
T= tall allele
t= short allele
b
T
DNA
replication
t
b
B
B
b
T
T
t
t
Ploidy: chromosomes don’t always come in pairs

Haploid = a cell with only one of each kind of chromosome

Diploid = a cell with two of each kind of chromosome

Triploid = a cell with three of each kind of chromosome
1n = 2
Number of
each type
of chromosome
2n = 4
How many
different
kinds of
chromosome
3n = 6
Total number of
chromosomes
A cell that is 2n = 4
Unreplicated chromosomes
Chromosome
Number of chromosomes: 4
A cell that is 2n = 4
Replicated chromosomes
Chromosome
Sister
chromatids
Centromere
Number of chromosomes: 4
Humans are diploid
23 pairs of chromosomes:
2n = 46
Overview of
Mitosis:
Parent cell
Mitosis
Replicated chromosomes
condense at the start of mitosis
Sister chromatids
separate and two
daughter cells are formed.
Parent cell and daughter cell contain the
same complement of chromosomes.
The cell cycle represents all phases
in the life of a cell

Interphase:
• DNA replication (S phase) must precede mitosis so
that all daughter cells receive the same complement
of chromosomes as the parent cell.
• The Gap phases separate mitosis from S phase. This is
the time when molecular signals mediate the switch in
cellular activity.

Mitosis: division of nucleus

Cytokinesis: division of cytoplasm
The cell cycle:
Functions of Mitosis

Mitosis produces genetically identical cells

Purpose
• Asexual Reproduction
• Growth and Development
• Tissue repair
Asexual Reproduction in Amoeba
Growth and development
1-celled zygote
(2n)
mitosis
Sand dollar embryo
Multi-cellular adult (2n)
Tissue renewal
Dividing bone marrow cells
give rise to new blood cells