Chapter 9: Meiosis

Pages 207-213

Sexual Reproduction

the union of 2 specialized sex cells (gametes) to form a single cell called a zygote Gametes usually come from 2 different parents Results in genetic variation among offspring

How is Chromosome Number Kept the Same with Sexual Reproduction?

Homologous Chromosomes - each chromosome in a somatic (body) cell has a partner chromosome Similar in size, shape, and position of centromeres

• The sex chromosomes are called X and Y• Human females have a homologous pair of X

chromosomes (XX)• Human males have one X and one Y

chromosome• The 22 pairs of chromosomes that do not

determine sex are called autosomes

• Each pair of homologous chromosomes includes one chromosome from each parent

• The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

• A diploid cell (2n) has two sets of chromosomes

• For humans, the diploid number is 46 (2n = 46)

• A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)

• For humans, the haploid number is 23 (n = 23)• Each set of 23 consists of 22 autosomes and a

single sex chromosome• In an unfertilized egg (ovum), the sex

chromosome is X• In a sperm cell, the sex chromosome may be

either X or Y

Gametes are Haploid

When egg and sperm join at fertilization, each gamete is haploid The diploid number is restored in the fertilized egg

(zygote). When the zygote divides by mitosis to form the

first 2 cells of the embryo, each daughter cell receives the diploid number of chromosomes

Polyploid

3 or more sets of chromosomes Rare in animals Common among plants

Important factor in plant evolution Larger and hardier than diploid Wheat, cotton

Meiosis

Diploid cells -> Haploid Cells Meiosis: 1 cell undergoes 2 divisions –

Meiosis I and II Produces 4 haploid cells Similar to mitosis, except:

2 divisions Only 1 DNA duplication (first division) Genetic information is shuffled

http://www.johnkyrk.com/meiosis.html

Meiosis

• In a cell in which DNA synthesis has occurred, each chromosome is replicated

• Each replicated chromosome consists of two identical sister chromatids

Prophase I Remember – the chromosomes are

duplicated during the S phase (before meiosis)

Homologous chromosomes synapse – line up side by side Form tetrads

Crossing over occurs – genetic material is exchanged between homologous chromatids Produces a new combination of genes Increases the genetic variation among offspring

Metaphase I

Kinetochores are attached by spindle fibers to the poles Both sister chromosomes are attached to the

same pole, the other sister chromosomes to the other pole

Tetrads (4 chromatids) line up on the equatorial plane of the cell

Tetrads are held together at chiasmata – sites of prior crossing-over

Anaphase I

Paired homologous chromosomes separate and move toward opposite poles

Each pole receives a random mixture of maternal and paternal chromosomes Only one member of each homologous pair is

present at each pole. Sister chromatids are united at their

centromere regions.

Telophase I

One of each pair of homologous chromosomes is at each pole.

Each new nucleus contains the haploid number of chromosomes, but each chromosomes is a duplicated chromosome.

Cytokinesis occurs: Often called interkinesis

Video

Meiosis II

Prophase II Metaphase II

Chromosomes condense again

NO crossing over occurs

Chromosomes line up on equatorial planes of their cells

Chromatids are in groups of two

Anaphase II Telophase II

Sister chromatids separate

chromosomes move to opposite poles

Nuclei form at opposite poles of each cell

Cytokinesis occurs

Results of Meiosis

Four n cells with unduplicated chromosomes Animals – production of gametes

“gametogenesis” Spermatogenesis or oogenesis

Plants – production of spores

Tutorial

Oogenesis

Human females have about 700,000 primary oocytes at birth

Suspended in Prophase I until puberty Usually 1 egg is ovulated per cycle ~ 28 days

Oocyte fully matures by finishing meiosis

Human female ovulates ~ 400 times/lifetime Meiosis II only finishes if fertilization occurs