Meiosis & Sexual Reproduction

Sexual Reproduction

• Involves:

Meiosis

Gamete production

Fertilization

• Produces genetic variation among offspring

Sexual Reproduction Shuffles Alleles

• offspring inherit new combinations of alleles, leading to variations in traits

• variation in traits is the basis for evolutionary change

Variation• Sexual reproduction results in

greater variation among offspring than does asexual reproduction.

• Two parents give rise to offspring that have unique combinations of genes.

• Offspring vary genetically from their siblings and from both parents

Meiosis

• function: production of gametes

• where: gonads of the mature adult; ovaries & testes.

• two consecutive nuclear divisions (Meiosis I & Meiosis II)

• DNA is NOT replicated during interkinesis

• result: 4 haploid (1n) daughter cells

Meiosis reduces the chromosome number from diploid to haploid.

Meiosis, like mitosis, is preceded by chromosome duplication…

…but, in meiosis the cell divides twice to form four daughter cells.

Meiosis I

• starts with synapsis, the pairing of homologous chromosomes

• crossing over homologous chromosomes exchange corresponding segments.

• each homologous pair separates producing two daughter cells, each with one set of chromosomes

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

– The stages of meiosis

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes (with centriole pairs)

Sites of crossing over

Spindle

Microtubulesattached to kinetochore

Metaphaseplate

Sister chromatids remain attached

Nuclearenvelope Chromatin

Sisterchromatids Tetrad

Centromere(with kinetochore)

Homologouschromosomes separate

Figure 8.14 (Part 1)

What are homologous chromosomes?

• Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with dyes.

• Homologous chromosomes carry genes that control the same traits.

Homologous Chromosomes

Crossing over increases genetic variability.

Genetic recombination results from crossing over during prophase I of meiosis and increases variation still further.

Tetrad

TE

M 2

,200

PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE IAND CYTOKINESIS

TELOPHASE IIAND CYTOKINESIS

Cleavagefurrow

Haploid daughter cellsforming

Sister chromatidsseparate

MEIOSIS II: Sister chromatids separate

Figure 8.14 (Part 2)

Meiosis II - Stages

Prophase II Metaphase II Anaphase II Telophase II

Fertilization

is the union of two haploid gametes; one from each parent.

Ex: in humans the egg and sperm each contain 23 chromosomes so after fertilization a zygote will have 46 chromosomes.

Fertilization/Zygote

Early Embryological Development