MEIOSIS

REPRODUCTION

• Asexual • splitting

• budding

• parthenogenesis (egg develops w/o fertilization) • occurs naturally in some invertebrate animal species

(e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles, and very rarely birds)

2

SEXUAL REPRODUCTION

• Fusion of two gametes to produce a single zygote.

• Introduces greater genetic variation, allows genetic

recombination.

• With exception of self-fertilizing organisms (e.g.

some plants), zygote has gametes from two

different parents.

3

REVIEW

• What happens in each of the phases of mitosis?

• Prophase

• Metaphase

• Anaphase

• Telophase

WHAT WOULD HAPPEN IF CELLS ONLY DID MITOSIS?

Cycle Number Stage Number of

chromosomes

1 Mitosis 2

1 Interphase

1 Fertilization

2 Mitosis

2 Interphase

2 Fertilization

3 Mitosis

3 Interphase

3 Fertilization

MEIOSIS

• Occurs in reproductive organs

• Meiosis produces haploid gametes

• ½ the number of chromosomes of regular

cells

CHROMOSOMES AND CHROMOSOME NUMBER

• DNA in chromosomes is arranged into

segments called genes

• Control production of proteins

• Each chromosome has hundreds of genes

HOMOLOGOUS CHROMOSOMES

• Human Somatic cells each have 46 chromosomes

• 23 from each parent

• 23 pairs of chromosomes

• Chromosomes that make up a pair = homologous

chromosomes

• Have same length and same centromere position

• Carry genes that control the same trait

• Don’t necessarily code for the same expression

of gene

HOMOLOGOUS CHROMOSOMES

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HAPLOID & DIPLOID CELLS

• Gametes – sex cells

• Have half the number of chromosomes as somatic

cells

• Humans – 23 individual chromosomes (not pairs)

• Haploid – n

• Sex cells

• Diploid – 2n

• Somatic cells

MEIOSIS I

• Interphase – same as always

• Prophase 1

• Chromatin condenses into chromosomes

• Homologous chromosomes pair up

• Crossing over occurs

• Spindle forms

CROSSING OVER

• Process in which chromosomal segments are

exchanged between a pair of homologous

chromosomes

• Metaphase I • Homologous

chromosomes line up in the middle of the cell

• Anaphase I • Homologous

chromosomes are pulled to opposite ends of the cell

• Telophase I

• Chromosome uncoils

• Nucleus reforms

• Cells split apart

• Results in 2 diploid

daughter cells

MEIOSIS II

• DNA IS NOT REPLICATED AGAIN!!!

• Prophase II

• Spindle forms

• Chromosomes condense

• Nucleus disappears

• Metaphase II

• Chromosomes line up in the middle

• Anaphase 2

• Sister chromatids are pulled to opposite ends of

the cell

• Telophase II

• Chromosomes reach poles

• Nuclear membranes reform

• Results in 4 haploid daughter cells

IMPORTANCE OF MEIOSIS

• Results in genetic variation

• How homologous pairs line up in metaphase I is

random

Meiosis Mitosis

Number of

divisions

Number of cells

formed?

Type of cells

formed

(haploid/diploid)

Where does it

occur?

Why does it

happen?

Unequal distribution of

chromosomes during

meiosis

Resulting gametes zero

or two copies of a

chromosome instead of

a single copy

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