Meiosis Chromosome Number n The number of cells

Meiosis

Chromosome Number n The number of cells per cell vary between organisms. Humans have 46 chromosomes, fruit flies have 8. n Humans have 23 different pairs of chromosomes, fruitflies have 4. n The number of different pairs is given as n, which is the haploid number of chromosomes

Haploid # Diploid # Humans n = 23 2 n = 46 Fruitflies n=4 2 n = 8

Homologous Chromosomes n Each partner in pair of chromosomes has the same basic shape and structure. They also contain the same exact genes, but may carry different alleles.

Human Karyotype

Creating a Karyotype

Mitosis - Refresher n Mitosis produces daughter cells having the same number of chromosomes as the parent cell 2 n n Produces genetically identical cells n One cell to two cells n Process used to replace old cells or dying cells

Meiosis n Process of nuclear division that reduces the number of chromosomes by half. n Used in sexual reproduction to make gametes (i. e. – sperm and egg cells) 1 n (diploid cells become haploid) n Two nuclear divisions 1 cell 4 cells n 2 n

Meiosis Phases

Prophase I n DNA strands thicken and coil n Spindle fibres appear n Nuclear membrane & nucleolus disappear Ø SYNAPSIS occurs (process by which each chromosome finds its homologue) Ø TETRAD formation (process by which homologous chromosomes line up with each other) Ø CROSSING-OVER – process whereby homologous chromosomes or sister chromatids twist around each other and exchange genes.

Crossing-Over

Crossing-Over

Crossing-Over

Metaphase I n The tetrads move to the equator of the cell n Homologous pairs (tetrads) remain together

Anaphase I n The homologous pairs of chromosomes separate n One chromosome is pulled to each pole. n Each chomosome is still composed of two chromatids joined by a centromere.

Telophase I n The cytoplasm divides, forming two daughter cells, with half the number of double-stranded chromosomes. Male n 2 n n Female

Meiosis II n Occurs in each cell formed during meiosis I and is NOT preceded by DNA replicaton

Prophase II n Chromosomes are coiled and new spindle fibres form

Metaphase II n Chromosomes n Each move to the cell equator. chromosome is made of two sister chromatids that are joined by a centromere that is attached to a spindle fiber.

Anaphase II n The centromeres joining the chromatids divide, freeing each sister chromatid from the other. n The sister chromatids move toward an opposite pole.

Telophase II Male n Spindle dissolves n Nuclear membrane reforms n Cytokinesis occurs Female

Meiosis – Result n The two nuclear divisions of meiosis result in four daughter cells from a single parent cell, each with half the chromosome number of each parent n 46 double stranded chromosomes 23 single stranded chromosomes

Law of Independent Assortment

Meiotic Accidents & Genetic Disorders

Nondisjunction during Meiosis n During this type of accident homologous chromosomes fail to separate during anaphase I or a double stranded chromosome fails to separate during anaphase II n Most nondisjunctional accidents are fatal and cause the embryo to spontaneously abort

Trisomy 21 – aka Down Syndrome n Most common autosomal nondisjunction accident n Contains a third chromosome 21 n The 21 st pair of chromosomes fails to separate during meiosis in the mother n Is not a fatal accident and therefore mother’s carry their baby to term

Characteristic Syptoms Include: • • • Notably round face Skin fold at the inner corner of the eye Flattened nose bridge Small, irregular teeth Short stature Heart defects Hightened susceptibility to respiratory infections Leukemia Alzheimer’s disease Various degrees of mental retardation Usually sterile, but some exceptions Half the egg cells will have two 21 chromosomes, so there is a 50 % chance of transmission to child

Incidence of Down Syndrome Compared to Maternal Age

Klinefelter Syndrome (XXY) n n n Reduced testes Sterile Breast development Normal intelligence Also found in other individuals with more than one additional sex chromosome – XXYY, XXXXY (more chance of mental retardation)

Turner Syndrome (XO) n n n Short stature Sterility caused by underdeveloped ovaries Small breasts Webbed necks Normal intelligence Only syndrome where having only 45 chromosomes is not fatal

Chromosomal Mutations

Cri-du-Chat Syndrome n n n Deletion mutation in chromosome 5 Diagnosed usually by a characteristic cry in children that sounds like a distressed cat mewing Mental retardation Widely-spaced eyes Epicanthic folds The degree of disability is directly related to the amount of genetic material missing

Chronic Myelogenous Leukemia