Meiosis/ KaryotypesUnit 6

Part 1: Chromosomes

Differences among Species•Each species has a characteristic number

of chromosomes

Potatoes, plums, and chimpanzees all have 48 chromosomes

Oats, Raccoon Dogs, Rats, Wheat and Wolverines all have 42 chromosomes

Humans have 46 chromosomes

Chromosome-•Composed of DNA and protein.•Consist of 2 identical CHROMATIDS attached by a CENTROMERE when the cell is ready to divide (after S of interphase)(Before division, chromatids separate to ensure identical DNA in both)

Gene-•Units of information•Segment of DNA that codes for a protein or RNA molecule

Somatic Cells

•What are somatic cells?•Have 46 chromosomes in humans.

▫Differ in size, shape, and genes▫23 pairs

2 HOMOLOGOUS chromosomes that are similar in size, shape, and genes

One comes from each parent!•Diploid (2n)

▫Contains 2 sets of chromosomes▫In humans, 2n=46

Gametes•What are gamets?•Contain 1 set of chromosomes

▫23 in humans•Haploid (n)• In humans n=23

•Fusion of 2 gametes= FERTILIZATION▫Fertilized egg cell is a zygote▫23 chromosomes+23 chromosomes=46Chromosomes▫n+n=2n

Chromosomes

Autosomes:•22 of the 23 pairs of chromosomes in somatic cells (44 of 46 chromosomes)

Sex Chromosomes:•1 of the 23 pairs (2 of the 46 chromosomes)•XX or XY in humans•XO system for birds and butterflies

Karyotype - photograph of chromosomes grouped in order from largest to smallest in pairs; used to analyze chromosomes

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22

This is a human karyotype.

How many chromosomes are shown?

Was this made from the nucleus of a somatic cell or a gamete?

How many autosomes are shown?

How many sex chromosomes?

Is this from a male or a female?

How do you know?

Abnormal Karyotypes

Nondisjunction•Sometimes during meiosis, the

chromosomes fail to separate correctly (called nondisjunction)Trisomy- when the other gamete has

3 copies of one chromosome•Klinefelter’s syndrome:

• One to several extra sex chromosomes

• Ex. XXY or XXXY

Monosomy- when gamete has only 1 copy of the affected chromosome•Turner’s syndrome:

• Only one sex chromosomeNo Y= develops into female

• Ex. XO

Nondisjunction •Down’s Syndrome:

▫Autosomal▫Trisomy 21▫Most common birth

defect

•Patau Syndrome:▫Autosomal▫Trisomy 13▫Rarely live past infancy▫Neurological problems,

polydactyl and facial defects

Nondisjunction•Edwards Syndrome:

▫Autosomal▫Trisomy 18▫30% of babies die by 1

month▫Small head, Clenched

hands with 2nd and 5th fingers on top of the others, Learning disabilities, congenital heart defects and malformations of digestive tract, urinary tract and genitals

Meiosis

What is Meiosis?

•A form of cell division that produces sex cells (gametes) used in fertilization

How is Meiosis Different?

•There are 2 divisions in meiosis (Meiosis I and II)

• The result is 4 cells instead of 2• The ending number of chromosomes is 23 in

humans (egg has 23 and sperm has 23)Is this haploid or diploid?

Mitosis Vs. Meiosis• Mitosis

▫1 division= 2 cells▫Daughter cells

identical▫Diploid cells (2n)= 46▫Body/somatic cells

• Meiosis▫2 divisions= 4 cells▫Daughter cells different▫Haploid cells (n)=23▫Gametes (sex

cells=egg/sperm)

Why are the daughter cells in meiosis different??

Genetic Variation! •Crossing Over-

▫ During prophase I pieces of homologous chromosomes are exchanged.

Why are the daughter cells in meiosis different??Genetic Variation! • Independent

Assortment-▫Homologous

chromosomes are randomly sorted/distributed during metaphase I and II. (Which chromosomes

go to which cell is random)

Why are the daughter cells in meiosis different??

Genetic Variation! •Random fertilization- the fertilization of

an egg and sperm is random!