4.2 SEXUAL REPRODUCTION

SEXUAL REPRODUCTION requires 2 parents

produces genetically distinct offspring

fertilization: fusion of a male reproductive cell with a female reproductive cell. (gametes)

zygote

each gamete has one set of homologous chromosomes - haploid (n)

a cell with two sets is diploid (2n)

The human diploid number is 2n = 46.

MEIOSIS process that produces gametes (haploid)

2 outcomes:

1. genetic reduction: a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell

2. genetic recombination: the products of meiosis have different combinations of alleles

involves two complete cycles of the four phases

1. meiosis I: prophase I, metaphase I, anaphase I, and telophase I

2. meiosis II: prophase II, metaphase II, anaphase II, and telophase II

MEIOSIS I• interphase: replication of the DNA

• prophase I:

• homologous chromosomes line up side-by-side in an alignment called synapsis.

• some segments are exchanged, increases genetic diversity (genetic recombination)

• metaphase I: homologous chromosomes lined up together along the equator.

• anaphase I: homologous chromosomes are separated

• telophase I: homologous chromosomes uncoil and spindle fibres disappear

• cytokinesis: a nuclear membrane forms around each group of chromosomes, haploid cells

MEIOSIS II

• prophase II and metaphase II: similar to mitosis

• anaphase II: sister chromatids are separated

• telophase II and cytokinesis four haploid cells

Meiosis involves two

complete cycles of four

phases. Notice that each

cell contains some

chromosomes from the

mother (yellow), some

chromosomes from the

father (blue), and some

chromosomes with

segments that have been

exchanged (yellow and

blue).

Magnification: 200x

MEIOSIS

COMPARING MITOSIS AND MEIOSIS

GAMETE FORMATION IN ANIMALS Male:

Spermatogenesis: process that produces sperm in male animals

starting at puberty, the spermatogonia reproduce by mitosis and then meiosis

4 sperms

Female:

oogenesis: produces eggs in females.

the oogonia reproduce by mitosis before birth, begin meiosis, stop at prophase I.

after puberty, each month, one cell completes meiosis.

cytoplasm is unequally distributed, and only one cell matures, 1 egg (not four).

MULTIPLE BIRTHS

GENETIC VARIATION

TWO ways:

1. independent assortment

2. crossing over

INDEPENDENT ASSORTMENT gametes carry different combinations of maternal and paternal chromosomes

during metaphase I, each homologous chromosome is randomly oriented towards one of the poles

223 over 8 million different chromosome combinations.

CROSSING OVER

genetic material between maternal and paternal chromosomes is exchanged

during prophase I, non-sister chromatids exchange genetic material in multiple sections.

ERRORS IN CHROMOSOME STRUCTURE during crossing over, chemical bonds are re-formed

errors can occur that can result in the following:

ERRORS IN CHROMOSOME NUMBER

non-disjunction: homologous chromosomes or sister chromatids do not separate during meiosis (anaphase I or anaphase II)

produces gametes that have too many or too few chromosomes

Eg. Trisomy 21 (three copies of chromosome 21) Down Syndrome

Monosomy is the term used for a single chromosome copy error.

ERRORS IN CHROMOSOME NUMBER

PRENATAL GENETIC TESTING

all ages may request or be referred

older more frequent

possibility of pregnancy termination with some procedures

HOMEWORK

p.181 #1-8, 11