Topic 3 Meiosis (gamete formation)

Human chromosomes Human somatic cells (any cell other than a gamete) have: 22 pairs of homologous chromosomes 1 pair of sex chromosomes 46 individual chromosomes Individuals inherit one set from each parent Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Human chromosomes 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, which is much smaller than an X chromosome The 22 pairs of chromosomes that do not determine sex are called …? Are these chromosomes from a male or female? How do you know? Sex chromosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

How we describe cellular chromosome number l ll ll Diploid (2n)Haploid (n) Chromosome # = 6Chromosome # = 3 l l l l Cells with 1 complete set of chromosomes (only one of each gene) Cells with 2 complete sets of chromosomes (1 set from mom, 1 set from dad – pairs!…this means that you have 2 versions of every gene.) HumansHuman gametes (sex cells)

n=23 egg sperm n=23 2n=46 zygote 5 Human gametes (n) are combined for reproduction. The fertilized egg is now 2n and is called a zygote. Fertilization

Meiosis & Sexual Reproduction

Figure 13.7 The stages of meiotic cell division: Meiosis I OBJ 15 7

Figure 13.7 The stages of meiotic cell division: Meiosis II OBJ 15 8

2nd division of meiosis separates sister chromatids 1st division of meiosis separates homologous pairs Double division of meiosis DNA replication Meiosis 1 Meiosis 2

Trading pieces of DNA creates variation Crossing over  during Prophase 1, sister chromatids of homologous chromosomes intertwine homologous pairs swap pieces of chromosome DNA breaks & re-attaches tetrad prophase 1

Variation from: Independent assortment of chromosomes  gametes of offspring do not have same combination of genes as gametes from parents random assortment in humans produces 2 23 (8,388,608) different combinations in gametes from Dadfrom Mom offspring new gametes made by offspring

Mitosis vs. Meiosis

Gametogenesis in humans Oogenesis: formation of eggs Spermatogenesis: formation of sperm Occurs in the reproductive organs

Oogenesis  During fetal development, oocyte pauses after Prophase I  Meiosis 1 completed during maturation  Meiosis 2 pauses in Metaphase II  Meiosis 2 completed after release from ovary

Sperm production

Variation from random fertilization Sperm + Egg = ?  any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences.

When meiosis goes wrong: chromosomal disorders

Human chromosomes review 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, which is much smaller than an X chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes Sex chromosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chromosome features Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters The banding pattern results from the coiling of the DNA and the histone proteins Banding patterns are unique to individual chromosomes. Homologous chromosomes with the same genes have the same banding pattern. Copyright © 2010 Genetic Science Learning Center, University of Utah

ACTIVITY INTERLUDE

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Karyotype review A karyotype is an ordered display of the pairs of chromosomes from a cell Karyotypes can tell us the sex of a fetus Karyotypes can also tell us if there are chromosomal abnormalities Autosomal disorders result from extra autosomal chromosomes Sex chromosome disorders result from extra sex chromosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Chromosomal disorders Result from errors during meiosis  Abnormal number of chromosomes (nondisjunction) Chromosomes fail to separate (either during meiosis 1 or meiosis 2) Usually fatal if an autosome Usually nonfatal if a sex chromosome (if there is at least one X)  Missing bits of chromosome (deletion)  Crossing over with nonhomologous chromosomes (translocation)

ACTIVITY INTERLUDE

Patient A

Patient B

Patient C

What’s the diagnosis? Patient A: Klinefelter’s syndrome  Extra sex chromosome (XXY) Patient B: Patau syndrome (trisomy 13)  Extra autosomal chromosome 13 Patient C: Down’s syndrome (trisomy 21)  Extra autosomal chromosome 21

Variation from: Crossing over: creates completely new combinations of traits on each chromosome  Can create an infinite variety in gametes

Sperm production Epididymis Testis Coiled seminiferous tubules Vas deferens Cross-section of seminiferous tubule spermatozoa spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) germ cell (diploid) MEIOSIS II MEIOSIS I

Oogenesis MEIOSIS I MEIOSIS II first polar body second polar body ovum (haploid) secondary oocyte (haploid) primary oocyte (diploid) germinal cell (diploid) primary follicles mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum developing follicle fertilization fallopian tube

Sexual reproduction introduces genetic variation  genetic recombination independent assortment of chromosomes crossing over  random fertilization Providing variation for natural selection The value of sexual reproduction metaphase1