Reproduction Mitosis produces somatic cells (body cells) Somatic cells have 46 chromosomes (23 pairs) What if somatic cells were involved in reproduction? 46 92 46 Oocyte Zygote Spermatocte I’ve got too many chromosomes!!! How do we solve this problem?
Reproduction Mitosis produces somatic cells (body cells) Somatic cells have 46 chromosomes (23 pairs) What if somatic cells were involved in reproduction? 23 46 Much better Haploid Diploid
Gametes Haploid gametes keeps the chromosome number from doubling in each succeeding generation Haploid gametes are produced by a special sort of cell division called meiosis Meiosis – occurs only in reproductive organs, ovaries and testes – Purpose of meiosis is to produce sperm and egg Spermatogenesis – Sperm production Oogenesis – Egg cell production
Meiosis involves 2 cell divisions Meiosis produces 4 cells from 1 parental cell Each of the 4 daughter cells has 23 individual chromosomes rather than 23 pairs of chromosomes Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis, is preceded by chromosome duplication – However, in meiosis the cell divides twice to form four daughter cells Summary of Meiosis
MEIOSIS I : Homologous chromosomes separate INTERPHASEPROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Sites of crossing over Spindle Sister chromatids Tetrad Microtubules attached to kinetochore Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate
MEIOSIS II : Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming
Genetic Variation in Meiosis Meiosis provides for a vast amount of genetic variation – Crossing over – in prophase I, chromosomes exchange DNA – Independent Assortment – in anaphase I homologs independently separate, in anaphase II sister chromatids independently separate Additional genetic variation is achieved through mating of non-related individuals
POSSIBILITY 1POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1Combination 2Combination 3Combination 4 Independent Assortment
The number of genetically distinct gametes that can be produced from a diploid cell is 2 n n = the number of chromosome pairs I wonder how many distinct gametes can be produced from independent assortment in humans? 2 23 = 8,388,608 combinations
MITOSISMEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE I Crossing over occurs PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE Chromosome replication 2n = 4 ANAPHASE TELOPHASE Chromosomes align at the metaphase plate Homologous pairs line up at the equator METAPHASE I ANAPHASE I TELOPHASE I Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I ; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II 2n2n2n2n Daughter cells of mitosis Daughter cells of meiosis II MEIOSIS II Daughter cells of meiosis I Haploid n = 2 nnnn
Karyotype A karyotype is a display of a persons chromosomes What does this karyotype tell you about the person?
Errors in Meiosis Nondisjunction occurs when either: – homologues fail to separate during anaphase I of meiosis, or – sister chromatids fail to separate during anaphase II. Trisomy - one extra chromosome (2n +1) Monosomy - one missing chromosome (2n - 1)
Down syndromeDown syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely: - characteristic facial features, short stature; heart defects - susceptibility to respiratory disease, shorter lifespan - prone to developing early Alzheimer's and leukemia - often sexually underdeveloped and sterile, usually some degree of mental retardation. - Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father's chromosome 21.
Patau syndrome (trisomy 13)Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months.
Edward's syndrome (trisomy 18)Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months.
Klinefelter syndrome: 47, XXY males. Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristics. Normal intelligence.Klinefelter syndrome: 47, XXY
47, XYY males47, XYY males: Individuals are somewhat taller than average and often have below normal intelligence. At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time.
Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotypeTrisomy X: 47, XXX
Monosomy X (Turner's syndrome)Monosomy X (Turner's syndrome): 1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)