Biology - Meiosis

Meiosis (definition)

• the division of the nucleus to produce 4 non-identical daughter nuclei

• cells produced through meiosis are genetically diverse

Mitosis vs. Meiosis

• Mitosis → produces somatic cells

  • cell divison for asexual reproduction, growth, and repair uses mitosis

  • cells produced are genetically identical

• Meiosis → gametes are produced

  • cell division for sexual reproduction uses meiosis

  • the division of the nucleus to produce 4 non-identical daughter nuclei

  • cells produced through meiosis are genetically diverse

Chromosome Terminology (Chromosome: Condensed)

• good for transport

• Present in Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis I, Prophase II, Metaphase II, and Anaphase II

Chromosome Terminology (Chromatin)

• good for reading

• uncondensed

• Present in G1, S, G2, Telophase II, Cytokinesis II

Locus (pl. loci)

specific location of a gene within a chromosome

Alleles

one of two or more alternative version of a gene at a locus (variants of a gene)

autosome

a chromosome that does not determine biological sex (22 pairs)

sex chromosome

• a chromosome that determines biological sex; humans have one pair (XX or XY).

• Male → one X and one Y

• Female → two X chromosomes

homologous pair of chromosomes

two chromosome with the same size, shape, position of the centromere, and types of genes

Tetrad

4 homologous sister chromatids

sister chromatids

• identical copies of a single chromosome joined together by a centromere (region w/ proteins in it)

• Daughter chromosomes → when sister chromatids split at the centromere, forming two independent and identical chromosomes

Karyotype

an ordered display of magnified images of an individual’s chromosomes arranged in pairs

types of cells (somatic, gamete, zygote, ploidy)

• somatic cell → body cell (46 chromosomes)

• sex cell/gamete → cell w/ only 23 chromosomes

• zygote → fertilized egg w/ 46 chromosomes (23 from egg and 23 from sperm)

• Ploidy → refers to the number of sets of chromosomes in a cell. For example, haploid (n) cells have one set, while diploid (2n) cells have two sets.

gonads

• glands responsible for making gametes

• in ovaries, meiosis produces “egg” cells

• in testes, meiosis produces sperm cells

Meiosis Phases (take this card to practice diagrams)

Practice Diagrams

Genetic Variation (Crossing Over)

• a physical exchange of genetic material between sister chromatids on homologous chromosomes occurring in prophase I of meiosis

1. Synapsis → homologs line up to form a tetrad

2. Breakage of nonsister chromatids

3. Nonsister chromatids joined by a chiasma (physical cross-over point btw homologs)

4. Separation of homologs at anaphase I

5. sister chromatids are separated at anaphase II, completing meosis

• parental type chromosome → contains original allele combinations prior to crossing over

• recombinant chromosome → contains a new combination of alleles bc crossing over

Genetic Variation (Independent Orientation of Tetrads)

• all the tetrads line up on the metaphase plate during metaphase I

• different possibilities for how they can be arranged on the metaphase plate

• because different cells have different orientations of tetrads → different cells produce different allele combinations in resulting gametes = genetic variation

Random Fertilization

random sperm fertilizes with a random egg

Chromosomal Anomalies (Nondisjunction)

• error in meiosis, in which chromosomes fail to separate properly

• can occur in Meiosis I or Meiosis II

  • If it’s in Meiosis I, then the gamete is (n+1, n+1, n-1, n-1)

  • If it’s in Meiosis II, then the gamete is (n+1, n-1, n, n)

• Typically, an abnormal number of autosomes is lethal except for Trisomy 21 (3 copies of #21), which causes Down syndrome

• Typically, an abnormal number of sex chromosomes is not lethal

Chromosomal Anomalies (Mutations)

• chromosomal deletion → loss of a piece of a chromosome

• chromosomal duplication → a piece of a chromosome can be deleted from one homolog and inserted in the other

• chromosomal inversion → a broken piece of a chromosome can flip and reinsert itself back into the broken chromosome

• chromosomal translocation →a piece of a chromosome can be deleted from one chromosome and inserted into a non-homologous chromosome

  • reciprocal translocation → the breaking of two non-homologous chromosomes and reinsertion in the other