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

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