MITOSIS: Brief Review  Occurs in somatic cells Somatic cells are the body cells (non-sex cells)  Purposes: replacement of damaged cells, growth of organism  Daughter cells are identical to parent cell  Each division of parent cell produces two daughter cells

Mitosis: Brief Review  In mitosis, chromosome number of offspring is the same as that of the parent

Chromosome Number  Somatic cells are DIPLOID Each cell contains 2 copies of each chromosome (one from each parent) Diploid = 2n

Human Chromosomes  Each somatic cell contains 46 chromosomes 22 pairs of autosomes  AUTOSOMES = non-sex chromosomes 2 sex chromosomes  pair of X’s (female)  An X and a Y (male)

Reproduction of organisms  Allows for survival of a species  Reproduction can be accomplished in different ways, depending on the organism  Reproduction is regulated by external forces (environmental conditions) and internal forces (hormones) These forces are not mutually exclusive

Asexual Reproduction  Results in a CLONE A clone is genetically identical to its parent  Advantage: only requires a single parent  Disadvantage: doesn’t allow for genetic recombination

Asexual Reproduction  Includes: Binary fission Fragmentation budding

Budding Hydra

Sexual Reproduction  Allows for gene recombination Important in survival of species “Survival of the fittest”  There needs to be a mechanism that allows for combination of genes but maintains the normal chromosome number Since 2 parents contribute chromosomes, the reproductive cells must have half the usual # of chromosomes

MEIOSIS  The process by which chromosome number is reduced by half.  Results in production of sperm and egg (gametes)

Chromosome Number  Gametes (egg and sperm) are HAPLOID Each contains just one set of chromosomes Haploid = n In humans, haploid number is 23  Remember: diploid number is 46

HUMAN CHROMOSOMES  46 CHROMOSOMES IN SOMATIC CELLS: 22 homologous pairs plus 2 sex chromosomes

Karyotype  Arrangement of homologous pairs of chromosomes  Used in genetic counseling or to diagnose a baby’s condition.

HOMOLOGOUS CHROMOSOMES

Homologous chromosomes  Are called HOMOLOGUES One is from male parent; one is from female parent

MEIOSIS  “reduction division” gametes have half the diploid number of chromosomes

Fertilization  The combination of gametes (egg and sperm  Results in a ZYGOTE  Restores the diploid number

MEIOSIS  CONSISTS OF 2 SUCCESSIVE DIVISIONS: MEIOSIS I, MEIOSIS II  ALLOWS FOR EXCHANGE OF GENETIC MATERIAL

Meiosis

Synapsis of Prophase I

CHROMOSOMES  Once DNA has replicated, the chromosomes are composed of 2 sister chromatids, joined by a centromere

TETRAD  A replicated pair of homologous chromosomes

Crossing Over  Genetic information is exchanged between homologous chromo- somes.  Chiasma (plural = chiasmata) is site of crossing over

Crossing Over Can you identify the chiasmata?

Meiosis: Summary

Meiosis: A summary  2 nuclear divisions resulting in 4 haploid cells  Random mixture of paternal and maternal genes This occurs by crossing over of homologues Occurs in Prophase I

Meiosis