Meiosis

Overview of Meiosis

Occurrence and Purpose

Exclusive to sexual reproduction

Occurs in specific cells in ovaries and testes

Generates offspring with unique trait combinations

Reduces chromosome number by half

Homologous Chromosomes

Cells (except gametes) have 23 pairs of chromosomes

Homologous chromosomes from each parent carry different gene versions

Genes are sequences of nucleotides on chromosomes

Karyotype and Sex Chromosomes

Karyotype displays magnified chromosome images in pairs

First 22 pairs are autosomes, last 2 determine gender

Sex chromosomes determine individual gender

Genetic Material Reduction

Haploid vs. Diploid

Diploid: 2 sets of chromosomes per cell (1 from each parent)

Haploid: 1 set of chromosomes per cell

Meiosis results in 4 haploid daughter cells

Gamete Formation

Gametes are sex cells produced by meiosis

Fusion of gametes at fertilization forms a zygote

Zygote develops into a new individual

Mechanisms of Meiosis

Cell Division Process

Meiosis involves two divisions without DNA synthesis in between

One chromosome duplication leads to two cell divisions

Four haploid daughter cells are produced

Stages of Gamete Production

Interphase precedes meiosis I for DNA replication

Meiosis I and II result in haploid daughter cells with reduced chromosome numbers

Detailed Phases of Meiosis

Meiosis I

Separation of homologous chromosomes

Chromosome duplication occurs like in mitosis

Synapsis aligns homologues before separation

Two haploid cells are formed

Meiosis II

Separation of sister chromatids

No DNA replication between Meiosis I and II

Similar to mitosis in process

Four haploid daughter cells are produced

Meiosis Process

Cell Division in Meiosis

Prophase II - nuclear membranes break down

Metaphase II - chromosomes line up

Anaphase II - sister chromatids separate

Telophase II - new nuclear membranes form

Cytokinesis - cytoplasm divides forming a total of 4 cells

Genetic Variation in Meiosis

Meiosis produces haploid cells not identical to parent cell

Crossing over - exchange of homologous parts between non-sister chromatids

Independent assortment of homologues - random separation of homologous chromosomes in meiosis I

Haploid cells resulting from meiosis have unique genetic compositions

Crossing Over

Synapsis - tight pairing of homologous chromosomes

Chiasmata - visual result of crossing over

Genetic recombination due to crossing over

Non-sister chromatids exchange genetic material

Genetic Variation in Meiosis

Independent Assortment

Chromosomes line up independently during metaphase I

Random separation of homologous chromosomes in anaphase I

Generation of gametes with a mix of maternal and paternal chromosomes

Ensures offspring are genetically different from parents

Meiosis I & II

Meiosis I ends with 2 cells, each with half the chromosomes of the parent cell

Meiosis II results in 4 haploid cells

Genetic variability in offspring due to combining chromosomes from different gametes

Meiosis ensures genetic diversity, a key advantage of sexual reproduction

Comparison of Mitosis & Meiosis

Key Differences

Meiosis requires two divisions, mitosis only one

Meiosis produces four haploid daughter cells, mitosis produces two diploid cells

Daughter cells from meiosis are genetically variable, while those from mitosis are identical

Meiosis introduces genetic diversity, crucial for evolution and adaptation

Unique Events in Meiosis

Crossing over in prophase I

Formation of tetrads at metaphase I

Separation of homologous pairs in anaphase I

Distinct processes in meiosis contribute to genetic variability

Genetics and Cell Division

Haploid and Diploid Cells

Haploid cells have a single set of unpaired chromosomes, like human eggs and sperm, each with 23 chromosomes.

Somatic cells are diploid, containing two sets of chromosomes, while gametes are haploid.

Human life cycle involves the production of haploid gametes by males and females, leading to the formation of a diploid zygote through fertilization.

Meiosis Process

Meiosis reduces the chromosome number from diploid to haploid to ensure the restoration of the diploid number during fertilization.

Distinct differences from mitosis include two rounds of division, resulting in four haploid cells with non-identical DNA in gametes.

Meiosis I involves prophase with synapsis and crossing over, metaphase with random alignment, anaphase with chromosome pairs separation, and telophase with cytokinesis.

Meiosis II leads to the formation of four haploid cells through prophase, metaphase, anaphase, and telophase followed by cytokinesis.

Key Meiosis Processes

Prophase I

Chromosomes condense, synapsis occurs with crossing over, leading to genetic variation.

Metaphase I shows chromosomes aligning as tetrads for random gene mixing through independent assortment.

Anaphase I and Telophase I

Anaphase I involves chromosome pairs moving to opposite poles, ensuring genetic diversity in gametes.

Telophase I results in two haploid daughter cells with one chromosome set each after cytokinesis.

Meiosis II Process

Prophase II, metaphase II, anaphase II, and telophase II lead to the formation of four haploid cells.