Sexual reproduction, meiosis, genetic variation

Meiosis and genetic variation

1. Independent assortment - leads to multiple possible paternal and maternal gene combinations bc it is random. Occurs in late prophase I, but homologous chromosomes separate in anaphase I. Homologous chromosomes come together to form tetrads. 2. Crossing-over of nonsister chromatids- late prophase I, chips a formation and synapsis, homologous chromosomes cross over. Random exchanging of alleles means genetically recombinant chromosomes. The four resulting sex cells will have different alleles. 3. Reduction to haploid - random separating of already genetically-different sister chromatids in anaphase II.

Human gametes

23 chromosomes, 22 are autosomal, 1 is a sex chromosome

Human Somatic cells

46 chromosomes, 44 are autosomal, 2 are sex chromosomes

Homologous chromosomes

Contain DNA that codes for the same genes in the same locations but different versions of those genes can be represented. One chromosome per homologous pair is from the dad and one is from the mom. In meiosis, homologous chromosomes separate, resulting in haploid cells.

Benefits of asexual reproduction

Faster, more efficient, sustainable, guaranteed supply of cells. Good for healing injuries and wounds.

Costs of asexual reproduction

Lack of genetic diversity means susceptible to environmental changes.

Biological role of sexual reproduction

Leads to genetic diversity that help organisms survive in changing environments. Meiosis splits the diploid number in half so that when two gamete cells fuse in fertilization, the resulting zygote is diploid, not tetraploid.

Differences between meiosis and mitosis at metaphase

Metaphase- chromosomes line up in the center of the cell, which leads to two diploid cells. Metaphase I - homologous chromosomes line up randomly on either side of the metaphasic plate, which leads to independent assortment and two haploid cells with replicated chromosomes, genetic variation with different alleles.

Karyotype

Micrograph of the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.

Q 12 on review sheet

Mitosis - 8 diploid ; meiosis - 4 haploid

Benefits of sexual reproduction

More genetic diversity means more genetic solutions to environmental challenges.

Source of variation shared by both mitosis and meiosis

Nondisjunction

Female gametes

Oogonia undergo meiosis --> 1 haploid ovum (live) and 3 polar bodies (dieee). Cytoplasm divides unequally in oogenesis. Meiosis is arrested in prophase I at birth, arrested at metaphase II during periods (one egg is released per month), doesn't proceed to completion unless sperm fertilizes it. Puberty through menopause. Mitosis division of Oogonia stoops before birth. (Limited supply)

Costs of sexual reproduction

Slower, complicated, requires more energy, err quirks finding the "perfect" mate.

What type of cells undergo mitosis?

Somatic cells Mitosis is a form of asexual reproduction.

Male gametes

Spermatogonia undergo Meiosis --> 4 sperm cells each with 23 chromosomes Spermatogenesis occurs from puberty to death.

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The genetic sex of a child is determined by sex chromosomes, whether the zygote received one X and Y chromosome, or two X's

What type of cells undergo meiosis?

They undergo meiosis to form haploid gametes that will fuse with another haploid gamete to create genetically diverse offspring. Diploid germ cells undergo meiosis (Oogonia or spermatogonia)