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

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

How we describe cellular chromosome number

lll l l

Diploid (2n)

Haploid (n)

Chromosome # = 6 Chromosome # = 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.)

Humans Human gametes (sex cells)

n=23egg

sperm n=23

2n=46zygote

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 divisionof 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

223 (8,388,608) different combinations in gametes

from Dadfrom Mom offspringnew gametes

made by offspring

Mitosis vs. Meiosis

Gametogenesis in humans

Oogenesis: formation of eggsSpermatogenesis: formation of spermOccurs 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 (223 x 223) possible diploid combinations

Sexual reproduction creates variabilitySexual 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 chromosomesCopyright © 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 productionEpididymis Testis

Coiledseminiferoustubules

Vas deferens

Cross-section ofseminiferous tubule

spermatozoa

spermatids(haploid)

secondaryspermatocytes

(haploid)

primaryspermatocyte

(diploid)

germ cell (diploid)

MEIOSIS II

MEIOSIS I

Oogenesis

MEIOSIS I

MEIOSIS II

first polar body

secondpolar body

ovum(haploid)

secondaryoocyte

(haploid)

primaryoocyte

(diploid)

germinal cell(diploid)

primary follicles

mature follicle withsecondary oocyte

ruptured follicle(ovulation)

corpus luteum

developingfollicle

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