Sec. 11-4 Meiosis

Chromosome Number

• 1/2 of the total # of chromosomes comes from the male and 1/2 of the total # of chromosomes comes from the female.
• A cell, which contains both sets of homologous chromosomes, is diploid.

              ü      Diploid means "two sets" or 2N

ü      In humans, the diploid number is 46 or 2N = 46

• The gametes of sexually reproducing organisms contain a single set of chromosomes is haploid.

ü      Haploid means "one set" or N.

ü      In humans, the haploid number is 23 or N = 23

Meiosis

• the reduction by division of the number of chromosomes by half in the sex cells (egg/sperm)

Meiosis I : The Reduction Division

• Before Meiosis I, each chromosome is replicated.

Prophase I

Early prophase

• Homologs pair
• Crossing over occurs

Late prophase

• Chromosomes condense
• Spindle forms
• Nuclear envelope fragments
• Crossing-over is an exchange of alleles between homologous chromosomes and produces a new combination of alleles.

Metaphase I

• Homolog pairs align along the equator of the cell.

Anaphase I

• Homologs separate and move to opposite poles.
• Sister chromatids remain attached at their centromeres.

Telophase I

• Nuclear envelopes reassemble
• Spindle disappears
• Cytokinesis divides cell into two

Meiosis II : The Equational Division

NEITHER CELL GOES THROUGH REPLICATION BEFORE MEIOSIS II

Prophase II

• Nuclear envelope fragments
• Spindle forms.

Metaphase II

• Chromosomes align along equator of cell

Anaphase II

• Sister chromatids separate and move to opposite poles.

Telophase II

• Nuclear envelope assembles
• Chromosomes decondense
• Spindle disappears

Cytokinesis divides cell into two.

Results of Meiosis

1)      Gametes

2)    Four haploid cells

3)    One copy of each chromosome

4)    One allele of each gene

5)    Different combinations of alleles for different genes along the chromosome

Spermatogenesis:  Sperm Formation

Ø      Stem cells in testes divide mitotically to create a pool of spermatocytes.

Ø      Meiosis produces four spermatids.

Oogenesis: Ovum (Egg) Formation

Ø      One of four meiotic products becomes an ovum.

Ø      The three remaining meiotic products are polar bodies.

Ø      Occurs in the ovaries

Ø      The mature egg has a rich storehouse of nutrients to nourish the young organism that develops after it is fertilized.

Fertilization

•         Fertilization is the joining of sperm and ovum.

•         Meiosis II in the ovum is completed at the time of fertilization forming one ovum and one polar body.

•         Following fertilization, chemical reactions occur preventing additional sperm from entering the ovum.

Human Heredity

Section 14-1 and 14-2

Human Chromosomes

•         Each chromosome is composed of a single, tightly coiled DNA molecule.

•         The two DNA strands are homologus (duplicates) and are held together by the centromere.

•         While they are still attached, the duplicated chromosomes are called sister chromatids.

•         Chromosomes can be categorized as two types:

v     Autosomes which are non-sex chromosomes that are the same number and kind between sexes.

v     Sex chromosomes which determine if the individual is male or female.

•         Sex chromosomes in the human female are XX and those of the male are XY.

•         Males produce X-containing and Y-containing gametes; therefore males determine the sex of the offspring.

•         A karyotype is a picture of chromosomes which are grouped together in pairs.

•         All animals have a characteristic number of chromosomes in the somatic or body cells called the diploid (or 2n) number.