Warm-up #7 1/23/15• Draw a chromosome and label the following parts: chromosome, sister

chromatid, centromere

• How many chromosomes do humans have?

Human Chromosomes• Humans have 23

pairs of chromosomes (46 total)

• 22 pairs of autosomes

• 1 pair of sex chromosomes

Karyotype – A picture of all the chromosomes in a somatic cell arranged by size

• One chromosome in the pair came from the male parent and one came from the female parent.

• Homologous chromosomes – each pair has genes for the same traits

Meiosisa type of cell division that results in four

daughter cells each with half the number of chromosomes of the parent cell

How can 2 cells join without doubling the amount of

chromosomes?

Meiosis: Where and Who?• For humans meiosis

occurs in the ovaries and testes

• The process of meiosis produces egg and sperm cells gametogenesis (spermatogenesis & oogenesis)

• Two gametes come together by fertilization

• Organisms produce gametes (sex cells) that contain one of each kind of chromosome.

• A cell with only one of each kind of chromosome is called haploid (n).

• Sex cells have one of each kind of chromosome so that when they combine – the resulting zygote is diploid

Meiosis: Why?

• Mitosis divides one diploid cell to form two diploid cells– For example: A human cell with 46

chromosomes divides to form two cells with 46 chromosomes.

• If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome– 46 chromosomes from each parent would yield

a 92 chromosome offspring• Meiosis allows for two divisions to divide

one diploid cell into four haploid cells.

Label the diagram

• Homologous chromosomes

• Sister chromatids

• Centromere• Chromosome

Homologous chromosomes

Chromosome

Centromere

Sister chromatids

Warm-up # 8 1/26/15

• How are homologous chromosomes different from sister chromatids?– Sister chromatids are identical– Homologous chromosomes have the

same genes, but may have different version of the gene

• What makes a cell haploid (n) or diploid (2n)?– Haploid – 1 set of chromosomes– Diploid – 2 complete sets of

chromosomes

Chromosomes

• Each chromosome can contain thousands of genes

• Genes can have different versions Alleles

• Meiosis I - homologous chromosomes separate

– Prophase I– Metaphase I– Anaphase I– Telophase I

• Meiosis II - sister chromatids separate (more similar to mitosis)

– Prophase II– Metaphase II– Anaphase II– Telophase II

Meiosis Phases

Interphase

• Before Meiosis (just like before Mitosis) the cell must prepare for division:– DNA and organelles are

replicated• During this phase,

chromosomes are not yet visible.– chromatin

Law of Segregation• Each gamete only gets one allele of

each gene

Independent Assortment• Mendel's law of independent assortment:

Genes for different traits assort independently of each other

• Organisms inherit two alleles for each trait when gametes are produced

Variation of Traits• Crossing over during Prophase I

– Exchange of genetic material between homologous chromosomes

Crossing Over

Humans have 223 different combination for gametes!

Mitosis Meiosis

Function

Type of cells

# of divisions

# of daughter

cells

Results in Haploid/di

ploid cells?

GrowthRepair

Reproduction

Create new cells for the purpose of reproduction

Somatic cells(body cells, anything other

than gametes)

Germ line cells (gametes/sex cells)

1 2

42

Diploid Haploid

Warm-up #9 1/27/15

• Name & describe 2 events/processes that occur during meiosis that contribute to genetic variation1. Crossing over – recombination of

alleles

2. Independent assortment – genes of different traits assort independently of each other

Warm-up #10 1/28/15

Copy the diagram in your notebook.

(1)Draw the possible gametes if crossing over DOES NOT occur

(2)Draw the gametes if crossing over DOES occur

(3)What affect does crossing over have on the variation of gametes?

Without Crossing Over

With crossing over

Law of Independent assortment

• 2n different combinations

• If we have 3 different chromosomes, how many different combinations are possible?

• n=3

• 23=8

Chromosomal Mutations• Errors can also occur during Meiosis. • Homologous chromosomes do not

separate properly –nondisjunction• Gametes with either an extra copy of a

chromosome or no copy

Norm

al

Exam

ple

Non

dis

jun

cti

on

E

xam

ple

s

Identifying Chromosomal Disorders

• Karyotype• Photograph is taken

of the paired chromosomes during metaphase

• Arranged according to length

• Easy to see if there are any extra or missing chromosomes

This individual has an extra Y

chromosome

Monosomy

• Zygote gets only one chromosome missing one chromosome– Most zygotes with monosomy do not

survive

Trisomy

• One copy of a chromosome from one parent and two copies from the other parent three copies

Downs Syndrome (Trisomy 21)

• 3 copies of the 21st chromosome• mental retardation, susceptibility

to certain illness or diseases, and a shorter life span