Meiosis

Sexual Reproduction and inheritance of characteristics

Asexual (vegetative) reproduction

• A form of duplication using only mitosis. • Example, a new plant grows out of the root or a

shoot from an existing plant. • Produces only genetically identical offspring since

all divisions are by mitosis. – Offspring called clones meaning that each is an exact

copy of the original organism – This method of reproduction is rapid and effective

allowing the spread of an organism – Since the offspring are identical, there is no mechanism

for introducing diversity

Sexual reproduction • Formation of new individual by a combination of two haploid

sex cells (gametes). • Fertilization- combination of genetic information from two

separate cells that have one half the original genetic information

• Gametes for fertilization usually come from separate parents – Female- produces an egg – Male produces sperm

• Both gametes are haploid, with a single set of chromosomes • The new individual is called a zygote, with two sets of

chromosomes (diploid). • Meiosis is a process to convert a diploid cell to a haploid

gamete, and cause a change in the genetic information to increase diversity in the offspring.

Chromosomes in a Diploid Cell

• Summary of chromosome characteristics Diploid set for humans; 2n = 46

• Autosomes; homologous chromosomes, one from each parent (humans = 22 sets of 2)

• Sex chromosomes (humans have 1 set of 2) – Female-sex chromosomes are homologous (XX) – Male-sex chromosomes are non-homologous (XY)

Karyotype

• A pictorial display of metaphase chromosomes from a mitotic cell

• Homologous chromosomes- pairs

What is meiosis I?

• In meiosis I, chromosomes in a diploid cell resegregate, producing two haploid daughter cells. It is this step in meiosis that generates genetic diversity.

The phases of meiosis I Prophase I

• DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. Chromosomal condensation allows these to be viewed in the microscope.

• Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

Prometaphase I

• The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

Metaphase I • Bivalents, each composed

of two chromosomes (four chromatids) align at the metaphase plate.

• The orientation is random, with either parental homologue on a side.

• This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Anaphase I

• Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles.

• Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Telophase I

• Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Cytokinesis

• Analogous to mitosis where two complete daughter cells form.

Meiosis II

• Meiosis II is similar to mitosis. However, there is no "S" phase.

• The chromatids of each chromosome are no longer identical because of recombination.

• Meiosis II separates the chromatids producing four daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.

Comparing Meiosis and Mitosis• Chromosome behavior – Mitosis: Homologous chromosomes independent – Meiosis: Homologous chromosomes pair forming bivalents

until anaphase I • Chromosome number- reduction in meiosis – Mitosis- identical daughter cells – Meiosis- daughter cells haploid

• Genetic identity of progeny: – Mitosis: identical daughter cells – Meiosis: daughter cells have new assortment of parental

chromosomes – Meiosis: chromatids not identical, crossing over

Meiotic errors• Nondisjunction- homologues don't separate in meiosis

1 http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm– Results in aneuploidy – Usually embryo lethal – Trisomy 21, exception leading to Downs syndrome – Sex chromosomes

• Turner syndrome: monosomy X • Klinefelter syndrome: XXY

• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Aneuploidy

• is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders (birth defects).

Turner syndrome

• As a result, girls with Turner syndrome (also known as Ullrich-Turner syndrome) tend to be sexually underdeveloped and infertile.

• Other common symptoms of Turner syndrome include a short stature and a webbed neck. About one in 2,500 girls has Turner syndrome.

Klinefelter syndrome

• Individuals with this syndrome are male, typically with the karyotype 47,XXY.

• They exhibit a characteristic phenotype including tall stature, infertility, gynecomastia and hypogonadism.

Translocation and deletion

Deletion

Insertion

Translocation

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently

• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).

• Trisomy 21 does not prevent meiosis

Animations

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

• http://www.stolaf.edu/people/giannini/flashanimat/celldivision/meiosis.swf