Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction

• Chapter 13 covers the processes of meiosis and sexual life cycles.

Meiosis

• Meiosis is a reduction division process vital for sexual reproduction.
• During this process, each individual gets genetic material from both parents.
• Individuals inherit chromosomes, where genes are located.
• Individuals will receive one set of chromosomes from each parent.

Key Terms

• Diploid cells are designated as "2n".
• Haploid cells are designated as "n".
• A gamete refers to a haploid reproductive cell, like sperm or egg.
• Fertilization combines two gamete cells (n).
• When fertilization happens, a zygote forms which is "2n".

Asexual vs Sexual Reproduction

• Asexual reproduction happens by only one parent.
• In asexual reproduction, one parent creates genetically identical offspring through mitosis.

Cell Types

• During meiosis, sex cells are created.
• Somatic cells are all other cells in the body except sex cells.

Chromosomes

• Sex chromosomes consist of one set of X and Y chromosomes.
• Autosomes consist of 22 sets of chromosomes.

Homologous Chromosomes

• Homologous chromosomes can be defined as two chromosomes composing a set.
• They have similar characteristics and genes but have different alleles.
• These are inherited with one chromosome from each parent.

Karyotype

• A karyotype is a visual representation of chromosomes in a cell.
• It screens for defective chromosomes or atypical numbers of chromosomes.
• Karyotypes come from isolated somatic cells that are then treated with drugs to stimulate mitosis.
• Cells are arrested in metaphase, then stained and viewed with a microscope and digital camera.

Meiosis Stages

• Meiosis involves two rounds of division.
• Homologous chromosomes separate during meiosis.
• Sister chromatids separate during meiosis.
• Interphase involves duplication of DNA during the S phase.
• Meiosis I separates homologous chromosomes and reduces the amount of chromosomes from diploid to haploid.
• Meiosis II separated sister chromatids and makes four haploid daughter cells.

Prophase I Feature

• During prophase I, homologous chromosomes pair together to form tetrads.
• Genetic information is exchanged with synapsis and crossing over.
• Chiasmata are regions of crossing over.

Cohesin Feature

• Sister chromatids are connected by cohesins, which are a special type of protein.
• Nonsister chromatids break at precisely corresponding spots.
• Synaptonemal complex holds homologs together.
• DNA breaks are repaired as DNA joins from one nonsister chromatid to the corresponding segment of another.

Separation of Homologs

• At anaphase I of meiosis, homologous pairs of sister chromatids move toward opposite poles of the cell.
• In anaphase II of meiosis, the sister chromatids separate, similar to mitosis.

Mitosis and Meiosis Comparison

• Mitosis occurs in somatic cells, whereas meiosis occurs in sex cells.
• Mitosis involves one division, whereas meiosis involves two.
• Genetic recombination doesn't occur in mitosis but it does during meiosis.
• Mitosis produces two diploid daughter cells that are genetically identical, while meiosis makes four haploid daughter cells which are genetically different

Genetic Variation

• Independent assortment occurs at metaphase I.
• During independent assortment, each pair orients independent of the others.
• Humans have 2 to the power of 23 possible combinations during independent assortment, equivalent to 8.4 million.
• During crossing over at prophase I, chiasmata results, and non-sister chromatids attach.
• During crossing over, genetic transfer happens.
• Random fertilization occurs as only one sperm cell fertilizes a single egg cell.
• 2 ^23 x 2 ^23 equals 70 trillion possibilities.

Evolutionary Significance of Genetic Variation

• Genetic variation is the raw material for evolution by natural selection within populations.
• Mutations are the original source of variance and persist when they confer survival and reproduction.
• Sexual reproduction makes new combinations of variant genes, increasing diversity.