SEXUAL REPRODUCTION AND MEIOSIS

Mitosis

Ensures that every body cell has 23 pairs of chromosomes; occurs during growth and repair of damaged cells

Meiosis

Involved in formation of gametes; produces haploid cells with 23 chromosomes

Importance of Meiosis

Reduces chromosome number and shuffles genes to produce genetically different gametes

Homologous Chromosomes

Same size

Diploid Cells (Humans)

Have 22 pairs of autosomes and 1 pair of sex chromosomes (46 total)

Sex Chromosomes

Males are XY; females are XX

Alleles

Different versions of the same gene that may code for different protein formsMeiosis I

Meiosis II

Sister chromatids separate into two cells each

Synapsis

Homologous chromosomes pair and line up side by side in meiosis I

Tetrad

Association of four chromatids (two homologous chromosomes with two chromatids each)

Crossing Over

Exchange of genetic material between non-sister chromatids

Genetic Recombination

Shuffling of alleles during crossing over to create diversity

Chiasma

Point where crossing over has occurred

Recombined Alleles

New genetic combinations formed during meiosis II when chromatids separate

Independent Assortment

Random separation of homologous chromosome pairs at metaphase I

Prophase I

Tetrads form; crossing over occurs; nuclear envelope fragments

Metaphase I

Tetrads align at spindle equator; either homologue can face either pole

Anaphase I

Homologues separate; dyads move to poles

Telophase I

Daughter nuclei are haploid

Interkinesis

Rest period between meiosis I and II; DNA replication does not occur

Prophase II

Chromosomes condense; nuclear envelope fragments

Metaphase II

Dyads align at spindle equator

Anaphase II

Sister chromatids separate into daughter chromosomes

Telophase II

Four haploid genetically different daughter cells are formed

Mitosis vs Meiosis II

Both separate sister chromatids; both produce daughter cells with the same number of chromosomes as the parent

Nondisjunction

Failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate

Down Syndrome (Trisomy 21)

Three copies of chromosome 21

Sex Chromosome Errors

Can result in XXX

Triplo-X (XXX)

Female with three X chromosomes; mild symptoms

Klinefelter Syndrome (XXY)

Male with extra X; symptoms include infertility and tall stature

Jacobs Syndrome (XYY)

Male with extra Y; often taller than average; usually no unusual features

Turner Syndrome (XO)

Female missing an X; short stature

Chromosomal Rearrangement

Can delete or duplicate genes

Inversion

Gene order is flipped

Translocation

Two non-homologous chromosomes exchange parts