How many chromosomes do humans have?

• Humans are

diploid

(2n) • Two of each chromosome, one from each parent.

• n = 23 unique chromosomes (haploid #) • 2(n)

=

46 total chromosomes Curly hair allele Strait hair allele • The two copies of each chromosome in human cells are

homologous

• Different versions - same genes in same locations but different DNA sequence. • Different versions (alleles) of a gene may promote different traits (e.g. hair type). 1

The diploid chromosome number in standard laboratory mice (genus Mus) is 40. What is the n for this organism (i.e. how many unique chromosomes does the mouse have?

2

Families: similar yet different. Why?

Asexual Reproduction

•Single parent •Offspring identical to each other and parent

Sexual Reproduction

•Two parents •Offspring are unique •Offspring are

similar

to each other and parents •Combine DNA from two individuals •Combines characteristics of both individuals 3

Comparison of Divisions

Number of divisions -

Mitosis (Asexual)

1

(Sexual)

2 Number of daughter cells Genetically identical?

2 Yes 4 No Chromosome # Same as Parent -Half of Parent Where When Role - Somatic (Body) Cells -Germ Cells (Gametes) - Throughout Life -At Sexual Maturity - Growth and Repair -Sexual Reproduction

4

Why do diploid organisms need to have specialized sex cells?

• Sex cells (gametes: sperm or egg) allow traits to be combined from two organisms

2n (46) + 2n (46) 4n = 92 too many

5

Sexual Reproduction

• Gametes have only one of each chromosome • Requires special cell division:

Meiosis

•Diploid cells (2n)

Gametes

(n) •Takes place in

gonads

(testis, ovary)

n (23) + n (23) 2n = 46

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How is Meiosis different from Mitosis?

Mitosis

Cells divide 1x

Meiosis

Cells divide 2x

2n 2n 2n 2n

Diploid Cells

n n n n n n

Haploid Cells 7

Mitosis Meiosis

Identical cells Different cells 8

Mitosis

Cell division is over

Meiosis

Cells divide again; sister chromatids line up Done 9

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What happens after meiosis (sperm and egg cells are made)?

Fertilization Forms a zygote!

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Why sexual reproduction?

Variation!? A Key To Survival!

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Why bother?

How many unique gametes can a cell with only two pairs of chromosomes make?

13

Why bother?

•For humans with 23 pairs of chromosomes?

•(2) 23 •For a couple > 8 million different possible gametes •possible unique offspring •(8 million) x (8 million) = (64,000,000,000,000)

But wait! There’s more!

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Crossing over enables even greater variety

• Exchange of equivalent sections between homologous chromosomes.

• Occurs at random locations along chromosome.

• Creates new versions of chromosomes.

15

Normal Meiosis Non-Disjunction

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Q: Does this person have a chromosomal abnormaility?

A. Yes B. No 17

How are chromosomal mutations detected?

Karyotyping

•Isolate chromosomes during fetal cell division. •Arrange in pairs according to size.

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