Sexual Reproduction and inheritance of characteristics 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

Translocation Deletion Insertion

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