Meiosis and Sexual Reproduction

Meiosis Meiosis

specialized cell division process produces haploid gametes

Each gamete receives 1 member of each pair of homologous chromosomes

Homologous Chromosomes

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Meiosis Separates Homologues

Meiosis consists of 1 round of DNA replication, followed by 2 rounds of nuclear divisions

These events occur in 2 stages: Meiosis I Meiosis II

Replication Before Meiosis Both members of

each homologous chromosome pair are replicated prior to meiosis

After replication, each chromosome consists of 2 sister chromatids

Meiosis I Each daughter cell

receives one member of each pair of homologous chromosomes

Meiosis II Sister chromatids

separate into independent chromosomes

Each daughter cell receives one of these independent chromosomes

Fusion of Haploid Gametes

Meiosis reduces chromosome number by half, producing 1n gametes (eggs and sperm)

Fusion of gametes (fertilization) combines two chromosome sets to produce diploid (2N) zygote

Overview of Meiosis I & II

Phases of meiosis have same names as the phases in mitosis, followed by I or II to distinguish the two nuclear divisions that occur in meiosis

Overview of Meiosis I

Meiosis I separates homologous chromosomes into 2 haploid daughter nuclei

Overview of Meiosis II

Meiosis II separates sister chromatids into 4 gametes

Meiotic Prophase I1. Homologous chromosomes pair up2. Crossing over (genetic recombination)

occurs between homologues Enzymes facilitate exchange of DNA

between arms of adjacent chromatids, producing chiasmata

3. Spindle microtubules assemble, nuclear envelope breaks down, and microtubules capture chromosomes

Meiotic Metaphase I

Duplicated homologous chromosomes are pulled into a line perpendicular to the spindle

Chromosomes line up as pairs of replicated homologous chromosomes

Meiotic Anaphase I

Meiotic Anaphase I

Homologous chromosome pairs separate

Each homologous chromosome pair moves to a pole, pulled by microtubules

Meiotic Telophase I

Meiotic Telophase I

Spindle microtubules disappear

Cytokinesis occurs

Nuclear envelopes may reappear

Chromosomes usually remain condensed

Meiosis II Meiotic Prophase II

Spindle microtubules reform and capture duplicated chromosomes

Each chromatid contains a kinetochore

Meiotic Metaphase II Duplicated chromosomes line up singly, perpendicular to the

spindle

Meiotic Anaphase II Chromatids separate

Meiotic Telophase II Cytokinesis occurs, nuclear membranes reform,

chromosomes relax

When Do Mitotic and Meiotic Cell Divisions

Occur in the Life Cycles of Eukaryotes?

Haploid Life Cycles• Fungi and unicellular

algae

• Most of life cycle is haploid

• Asexual reproduction by mitotic cell division produces a population of identical, haploid cells

• Life cycle - Chlamydomonas

Diploid Life Cycles• Most animals

• Most of cycle is in diploid state

• Haploid gametes are formed by meiosis

• Gametes fuse to form a diploid zygote

• Zygote develops into adult through mitotic cell divisions

Alternation-of-Generation Cycles

• Plants

• Includes both multicellular diploid and multicellular haploid body forms

• Multicellular diploid body gives rise to haploid spores, through meiosis

• Spores undergo mitosis to produce a multicellular haploid generation

Alternation-of-Generation Cycles

• Eventually, certain haploid cells differentiate into haploid gametes

• 2 gametes fuse to form a diploid zygote

• Zygote grows by mitotic cell division into a diploid multicellular diploid generation

How Do Meiosis and Sexual Reproduction Produce

Genetic Variability?

Novel Chromosome Combinations

• Genetic variability among organisms is essential in a changing environment

• Mutations produce new variation but are relatively rare occurrences

Novel Chromosome Combinations

• Randomized line up and separation of homologous chromosomes in Meiotic Metaphase I and Anaphase I increase variation

– # of possible combinations is 2n, where n = number of homologous pairs

Anaphase I

Metaphase I

Crossing Over

• Variation enhanced by genetic recombination

• Crossing over creates chromosomes with new allele combinations

• Combined with homologue shuffling in Metaphase/Anaphase I, each gamete produced in meiosis is virtually unique

Fusion of Gametes

• Fusion of gametes from 2 individuals further increases possible 2n combinations

• Gametes from 2 humans could produce about 64 trillion different 2n combinations

• Taken together with crossing over, each human individual is absolutely genetically unique

The End