Meiosis and Sexual Reproduction

Homologous Chromosomes Chromosomes of each pair are similar in length and centromere position Both carry genes controlling the same inherited characteristics We inherit one chromosome of each homologous pair from our mother and the other from our father…(Therefore, these are NOT identical!)

Meiosis A type of cell division that… – Occurs only in the sex organs Sperm cells in men Eggs in women – Produces 4 genetically unique cells each with HALF the number of chromosomes as the parent cell – Keeps the number of chromosomes in a species the same generation to generation

Karyotype

Body cells are called somatic cells. Germ cells develop into sex cells. – Gametes are sex cells: egg and sperm. – Gametes have DNA that can be passed to offspring Two Types of Cells

Haploid vs. Diploid Fertilization between egg and sperm occurs in sexual reproduction. Diploid (2n) cells have two copies of every chromosome. – Body cells are diploid. – Half the chromosomes come from each parent

Haploid (n) cells have one copy of every chromosome. –Gametes are haploid. –Gametes have 22 autosomes and 1 sex chromosome

Meiosis makes haploid cells from diploid cells

Some details of meiosis… As with mitosis, meiosis begins after the chromosomes have been duplicated during Interphase Meiosis consists of two distinct parts – Meiosis I – Meiosis II

Meiosis I Homologous chromosomes exchange genetic material (cross over) and separate during anaphase

Crossing over results in new combinations of genes

Meiosis II Sister chromatids are separated much as they are in mitosis except the resulting cells are haploid and not diploid

Key Differences between Mitosis and Meiosis Mitosis: – One cell division – No pairing of homologous chromosomes – Produces two offspring cells each with the same number of chromosomes as the parent cell (46) – Produces genetically identical copies of the parent cell Meiosis: – Two cell divisions – Homologous chromosomes pair – Produces four offspring cells each with one set of chromosomes—half the number as the parent cell (23) – Creates genetically unique cells different from the parent cell This is because homologous chromosomes exchange genetic information

Which process is this? Provides for growth, repair, and asexual reproduction Produces TWO daughter cells that are genetically identical to the parent cell Involves one division of the DNA Diploid parent cells produce diploid daughter cells Happens in most cells of the body

Which process is this? Yields haploid daughter cells with only ONE member of the homologous chromosome pair Produces FOUR daughter cells that are genetically different from the parent cell Involves TWO divisions of the DNA Exchange of genetic material occurs between homologous chromosomes

Gametogenesis All sperm and eggs produced during gametogenesis are genetically unique. What would happen if humans created gametes that were all genetically identical to each other? (Think about your siblings) Twins—identical vs. fraternal (how does this occur?)

Trisomy 21—Down’s Syndrome Three copies of chromosome 21 Maternal Age Incidence at Birth 201 in in in in in 30

Klinefelter’s Syndrome Male—XXY 1 in 1000 males Sterile Tall, thin, and slightly lower IQ

Other Disorders Turner’s Syndrome—XO – 1 in 500 female births (most are aborted before birth) – Very short, infertile, broad chest and may have a webbed neck Edward’s Syndrome— Trisomy 18 – 1 in 3000 – Most babies die within the first month to the first year