Most humans have 46 chromosomes (23 pairs) in their body cells.

They have only 23 chromsomes in their gametes.

Why do you think gametes only have half the number of chromsomes?

Talk to the people around you about what a Punnett square is

used for.

Talk to the people around you about the difference between

heterozygous and homozygous.

Talk to the people around you about the Law of Segregation.

Talk to the people around you about the Law of Independent

Assortment.

• Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes.

• Typically, a chromosome can contain a thousand or more genes along its length.

• Your body cells are diploid, and contain 46 chromosomes.

• A cell with two of each kind of chromosome is called a diploid cell.

Diploid Cells

• The two chromosomes of each pair in a diploid cell are called homologous chromosomes.

• Each pair of homologous chromosomes has genes for the same traits.

Homologous Chromosomes

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

• A cell containing one of each kind of chromosome is called a haploid cell.

Haploid Cells

Chromosome Numbers of Common Organisms

Organism Body Cell (2n)

Fruit fly 8Garden pea 14Corn 20Tomato 24Leopard Frog 26 13Apple 34Human 46Chimpanzee 48Dog 78Adder’s tongue fern 1260

Gamete (n)47

1012

17232439

630

Diploid and Haploid Cells

In Humans, how many chromosomes would each of these cells have…..

CHEEK CELL

46

SPERM CELL

23

EGG CELL

23

NERVE CELL

46

Since they have different amounts of chromosomes, the way they are made differ.

● Body cells are made by the division process MITOSIS.

● Sex cells are made by the division process MEIOSIS.

Before a cell can divide by either method, it must first prepare for cell division.

The cell cycle (or entire life cycle of the cell) consists of:

INTERPHASE: The longest/busiest stage in a cell’s life

1. Chromosomes (DNA) replicates

2. Cell grows bigger

CELL DIVISION: Mitosis or Meiosis

CYTOKINESIS: When the cytoplasm divides and the cell completely separates.

Mitosis: cell division that makes BODY CELLS.

It starts with a DIPLOID cell 46

Chromosomes Replicate

92

4646

It ends with TWO IDENTICAL DIPLOID cells

PHASES OF MITOSIS (Paul Makes A Taco)

PROPHASE

1. Chromosomes coil up

2. Nuclear envelope disappears

3. Spindle fibers form

METAPHASE

1. Chromosomes Line up in the middle

2. Spindle attaches to chromosomes

PHASES OF MITOSIS (Paul Makes A Taco)

ANAPHASE

1. Chromosomes split and are pulled apart

TELOPHASE

1. Chromosomes uncoil

2. Nuclear envelopes reappear

3. Cell is splitting into two

Meiosis: cell division that makes SEX CELLS.

It starts with a DIPLOID cell 46

Chromosomes Replicate

92

46 46

23 23 23 23

It ends with FOUR HAPLOID cells (not identical)

• These haploid cells are called sex cells— gametes.

• Male gametes are called sperm.

• Female gametes are called eggs.

Meiosis

• When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes.

Meiosis

Crossing-Over Crossing-Over

In Meiosis:

When homologous chromosomes actually break and exchange genetic material- is a process known as crossing over.

Crossing-OverCrossing-Over

Crossing over causes variations in the alleles on each chromosome.

• The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction.

•This would occur when homologous chromosomes are suppose to separate from each other.

Nondisjunction

• This chart of chromosome pairs is called a karyotype, and it is valuable in identifying unusual chromosome

numbers in cells.

• Down syndrome is the only autosomal trisomy in which affected individuals

survive to adulthood.

Down syndrome: Trisomy 21

Down syndrome: Trisomy 21

• The incidence of Down syndrome births is higher in older mothers, especially those over 40.

• Extra chromosome on 21

• 1 in every 1100 live births

• Range from mild to severe physical and mental disabilities

• Many abnormalities in the number of sex chromosomes are known to exist.

Abnormal numbers of sex chromosomes

• Any individual with at least one Y chromosome is a male, and any individual without a Y chromosome is a

female.

Turners Syndrome

• Absence of one X chromosome

• Occurs in 1 out of every 2,500 live female births.

• Physical characteristics

• No mental disability

Abnormal numbers of sex chromosomes

• Most of these individuals lead normal lives, but they cannot have children and some have varying degrees of mental retardation.

XXX

“Trisomy X”

XXY

“Klinefelter’s”

XYY

“Jacob’s Syndrome”

A cell with two of each kind of chromosome is __________.

Question 1

D. polyploid

C. biploid

B. haploid

A. diploid

The answer is A. The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each has genes for the same traits.

Homologous Chromosome 4

a A

Terminal Axial

InflatedD

Constrictedd

Tall

T

Short

t

What is the importance of meiosis in sexual reproduction?

Question 2Meiosis

Meiosis

Sperm Cell

Egg Cell

Haploid gametes

(n=23)

Fertilization

Diploid zygote

(2n=46)

Mitosis and Development

Multicellular

diploid adults

(2n=46)

Meiosis is cell division that produces haploid gametes. If meiosis did not occur, each generation would have twice as many chromosomes as the preceding generation.

Meiosis

Meiosis

Sperm Cell

Egg Cell

Haploid gametes

(n=23)

Fertilization

Diploid zygote

(2n=46)

Mitosis and Development

Multicellular

diploid adults

(2n=46)

Trisomy usually results from _______.(TX Obj 2; 6C)

Question 2

D. twenty-two pairs of chromosomes

C. nondisjunction

B. incomplete dominance

A. polygenic inheritance

The answer is C.