The instructions for making cell parts

are encoded in the DNA, so each new cell must get a

complete set of the DNA molecules

DNA must be copied or replicatedbefore cell division

Each new cell will then have an identical copy

of the DNA

OH

CH2

O

4

5

3 2

1

PO4

N base

ribose

Interphase = the period between cell divisions (made of 3 phases)

1) G1 = the growth phase, a new cell

grows to mature size

2) S = DNA is copied

or synthesized to prepare for division

1) G2 = the time when the cell prepares for

division

Each chromosome consists of two identical “sister” chromatids.

Each pair of chromatids is attached at an area called the centromere.

Chromosome = DNA (deoxyribonucleic acid) + associatedproteins (mainly histones) = “packaged” DNA

All eukaryotic cells store genetic information in chromosomes

Most eukaryotes have between 10 and 50 chromosomes in their

body cells

Human body cells have 46chromosomes or 23 identical pairs

Each chromosome is composed of asingle, tightly coiled DNA molecule

Chromosomes can’t be seen when cellsaren’t dividing and are called chromatin

DNA is tightly coiled around proteins called

histones

Duplicated chromosomes are called chromatids& are held together by thecentromere

G1 - primary growth phaseS – synthesis; DNA replicatedG2 - secondary growth phase

collectively these 3 stages are called interphase

M - mitosisC - cytokinesis

Cell Cycle

Division of the nucleusOnly occurs in eukaryotes

Has four stagesDoesn't’t occur in some cells

such as brain cells

1st growth stage after cell division

Cells mature by making more cytoplasm & organelles

Cell carries on its normal metabolicactivities

Synthesis stage

DNA is copied or replicated

2nd Growth Stage

Occurs after DNA has been copied

All cell structures needed for division are made (e.g. centrioles)

Both organelles & proteins aresynthesized

Division of thenucleusAlso calledkaryokinesisOnly occurs ineukaryotesHas four stagesDoesn’t occur insome cells such asbrain cells

Prophase

Metaphase

Anaphase

Telophase

Chromatin in nucleus condenses to form visible chromosomesMitotic spindle forms from fibers in cytoskeleton or centrioles (animal)

Chromosomes

Nucleolus

Cytoplasm

Nuclear Membrane

Nuclear membrane & nucleolus are broken down

Chromosomes continue condensing & are clearly visible

Spindle fibers called kinetochoresattach to the centromere of each chromosome

Spindle finishes forming between the poles of the cell

Nucleus & Nucleolus have disintegrated

Chromosomes

Kinetochore Fiber

Chromosome

Chromosomes, attached to the kinetochore fibers, move to the center of the cell

Chromosomes are now lined up at the equator

Pole of the Cell

Equator of Cell

Chromosomes lined at the Equator

Asters at the poles

Spindle Fibers

Occurs rapidly

Sisterchromatids are pulled apart to opposite polesof the cell by kinetochore fibers

Individual

chromosomes

Sister Chromatids being separated

Sister chromatids at opposite poles

Spindle disassemblesNuclear envelope forms around each set of sister chromatidsNucleolus reappearsCYTOKINESIS occursChromosomes reappear as chromatin

36

Have the same number of chromosomes as each other and as the parent cell from which they were formed

Identical to each other, but smaller than parent cell

Must grow in size to become mature cells (G1 of Interphase)

Preceded by interphase which includeschromosome replication

Two meiotic divisions --- Meiosis I and Meiosis II

Called Reduction- division

Original cell is diploid (2n)

Four daughter cells produced that are monoploid (1n)

Daughter cells contain half the number of chromosomes as the original cell

Produces gametes (eggs & sperm)Occurs in the testes in males (Spermatogenesis)

Occurs in the ovaries in females (Oogenesis)

Replication is the process of duplicating a chromosome

Occurs prior to division

Replicated copies are called sisterchromatids

Held together at centromere

Occurs in Interphase

Nucleus Spindlefibers

Nuclearenvelope

Early Prophase I(Chromosome number doubled)

Late Prophase I

Metaphase I

Anaphase I Telophase I (diploid)

Early prophaseHomologs pair.Crossing over occurs.

Late prophaseChromosomes condense.Spindle forms.Nuclear envelope fragments.

Homologous chromosomes

(each with sister chromatids)

Join to form a

TETRAD

Called Synapsis

Homologouschromosomes in a tetrad cross over each other

Pieces of chromosomes or genes are exchanged

Produces Geneticrecombinationin the offspring

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

Homologous pairs of chromosomes align along the equator of the cell

Homologs separate and move to opposite poles.

Sister chromatids remain

attached at their centromeres.

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

Only one homolog of each chromosome is present in the cell.

Meiosis II produces gametes with

one copy of each chromosome and thus one copy of each gene.

Sister chromatids carry

identical genetic

information.

Gene X

Prophase IIMetaphase II

Anaphase II

Telophase II

4 Genetically Different haploid cells

Nuclear envelope fragments.

Spindle forms.

Chromosomes align

along equator of cell.

Sister chromatidsseparate and move to opposite poles.

Equator

Pole

Nuclear envelope assembles.

Chromosomes decondense.

Spindle disappears.

Cytokinesis divides cell into two.

Gametes (egg & sperm) form

Four haploid cells with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome

Vesicles fuse to form cell wall and membranes

Complete separation of daughter cells

• During cytokinesis, the cytoplasm pinches in half.

• Each daughter cell has an identical set of duplicate chromosomes

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Mitosis Meiosis

Number of divisions

12

Number of daughter cells

2 4

Genetically identical?

Yes No

Chromosome # Same as parent Half of parent

Where Somatic cells Germ cells

When Throughout life At sexual maturity

RoleGrowth and

repairSexual reproduction

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