Key Terms: 11_Making Gametes and Zygotes

Meiosis (Primary)

Creation of haploid gametes. Includes recombination. Source of genetic variation between generations.

Sexual Reproduction

Creation of diploid zygotes. The final source of genetic variation between generations.

Zygote

Diploid cell formed when two haploid gametes fuse.

Gamete

Haploid reproductive cell produced by meiosis (for example, sperm or egg).

Sister Chromatids

The two copies of a replicated chromosome. Genetically identical as a result of semi-conservative DNA replication.

Ploidy

Number of sets of chromosomes.

Haploid

One set of chromosomes. Example

Diploid

Two sets of chromosomes. Example

Homologous Chromosomes

Chromosomes with the same sets of genes but possibly different sequences (alleles).

Mitosis (Contrast)

What most cells do when they divide (covered next unit). Meiosis is used exclusively for sexual reproduction and gamete formation.

Meiosis (Secondary)

One round of DNA replication followed by two rounds of cell division (Meiosis I and Meiosis II). One diploid parent cell creates four haploid gametes.

DNA Replication Before Division

Semi-conservative replication must occur before mitosis or meiosis.

Meiosis I

Separation of homologous pairs of replicated chromosomes.

Prophase I

Nucleus breaks down. Chiasmata present (crossing over). Crossing over occurs here.

Metaphase I

Homologous pairs line up in the center of the cell, with bivalents oriented randomly with respect to each other.

Bivalent

A homologous pair of replicated chromosomes aligned together.

Anaphase I

Homologous chromosomes separate, but sister chromatids do not separate.

Telophase I

Daughter cells are ready to move into prophase II.

Meiosis II

Separation of sister chromatids.

Prophase II / Metaphase II

Prophase II - nuclear envelope breaks down and chromosomes condense. Metaphase II - chromosomes align in the center of the cell.

Anaphase II / Telophase II

Anaphase II - sister chromatids separate. Telophase II - nuclear envelope re-forms. Result - haploid gametes.

Generating Variation 1 - Principle of Segregation

Homologous chromosomes are separated into gametes, so the 2 alleles of a single gene are separated. Segregation = single gene.

Generating Variation 2 - Principle of Independent Assortment

Differential alignment of homologous pairs in Meiosis I leads to different combinations of maternal and paternal chromosomes in gametes. Independent assortment applies to alleles of different genes.

Generating Variation 3 - Recombination (Crossing Over)

During Meiosis I, homologous chromosomes align and swap DNA segments between non-sister chromatids.

Crossing Over Components

Non-sister chromatids, sister chromatids, chiasma, paternal and maternal homologs, recombinant chromatids.

Synapsis

Pairing of homologous chromosomes during prophase I (alignment before recombination).

Synaptonemal Complex

Protein complex holding homologs together during synapsis, before crossing over.

Centromere

Region joining sister chromatids of a replicated chromosome.

Meiosis Order of Events

(1) Replication during interphase. (2) Synapsis during prophase I. (3) Crossing over during prophase I.

Crossing Over Frequency

Usually occurs at least once on each homologous pair (shown for two of four chromatids).

Crossing Over Consequence

Gametes with chromosomes that contain part of each of the original pair of homologous chromosomes.

Punnett Square (Primary)

Tool to keep track of possible allele combinations in gametes and zygotes. Axes list all possible gamete genotypes of each parent.

Punnett Square (Secondary)

Considers inheritance of one gene between two diploid parents.

Five Sources of Variation

Environmental damage, molecular machinery errors, transposons, meiosis and recombination (segregation, independent assortment, crossing over), and sexual reproduction (fusion of gametes into zygotes).