HOW MANY CHROMOSOMES ARE FOUND IN EACH HUMAN

BODY CELL?

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

• In the body cells of animals and most plants, chromosomes occur in pairs.

• In each pair, one you inherited from your mom, the other from your dad.

Diploid Cells

From Mom

From Dad

The two chromosomes of a pair are called homologous chromosomes.

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

Homologous Chromosomes

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

Diploid Cells

• Body cells (diploid) are different from sex cells.

• Each person can only pass on ONE of their homologous chromosomes to their young.

HUMANS = 46 CHROMOSOMES

IN THEIR BODY CELLS

Each parent passes 23 to their young, so that when the sperm and egg combine- it

will equal 46 Chromosomes.

• So, sex cells CANNOT be diploid.

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

Haploid Cells

Sexual Reproduction

Zygote

Chromosome Numbers of Common Organisms

Organism Body Cell

Fruit fly 8

Garden pea 14

Corn 20

Tomato 24

Leopard Frog 26 13

Apple 34

Human 46

Chimpanzee 48

Dog 78

Adder’s tongue fern 1260

Sex Cell

4

7

10

12

17

23

24

39

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

HomologsHomologsseparateseparate

Sister Sister chromatidschromatidsseparateseparate

DiploidDiploid

MeiosisMeiosis I I

MeiosisMeiosisIIII

DiploidDiploid

HaploiHaploidd

The kind of cell division that produces sex cells (haploid) containing half the number of chromosomes as a body cell, is called meiosis.

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)

Click here to see the animation http://www.cellsalive.com/meiosis.htm

• 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

Sect ion 10.2 Summary – pages 263-273

• Growth period in which the cell replicates its chromatin (DNA).

InterphaseInterphase

Meiosis I: Reduction DivisionMeiosis I: Reduction Division

NucleusNucleus SpindleSpindlefibersfibers NuclearNuclear

envelopeenvelope

Prophase IProphase I

Chromatin coils up.Chromatin coils up.

Spindle forms.Spindle forms.

Nuclear envelope Nuclear envelope fragments.fragments.

Tetrads Form in Tetrads Form in Prophase IProphase IHomologous Homologous

chromosomeschromosomes(each with sister chromatids) (each with sister chromatids)

  

Join to form aJoin to form a TETRADTETRAD

Crossing-Over in Crossing-Over in Prophase IProphase I

The tetrads pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

Crossing-OverCrossing-Over

Crossing over can occur at any location on a chromosome, and it can occur at several locations at

the same time.

• The centromere of each chromosome becomes attached to a

spindle fiber.

Metaphase IMetaphase I• In Metaphase I, the tetrads line up in the middle, or equator.

Anaphase IAnaphase I

Homologous Homologous chromosomes chromosomes separateseparate and move and move to opposite poles.to opposite poles.

Sister chromatids Sister chromatids remain remain attached attached at theirat their centromerescentromeres.

Telophase ITelophase I

Nuclear envelopes Nuclear envelopes reassemble.reassemble.

Spindle disappears.Spindle disappears.

Cytokinesis Cytokinesis divides cell divides cell into two.into two.

Meiosis II: Reducing Chromosome Meiosis II: Reducing Chromosome NumberNumber

Prophase Prophase IIII

MetaphaMetaphase IIse II

AnaphasAnaphase IIe II

TelophasTelophase IIe II 4 Identical 4 Identical

haploid haploid cellscells

The second division in meiosis is simply a mitotic division of the products of Meiosis I.

Prophase IIProphase II

Nuclear Nuclear envelope envelope fragments.fragments.

Spindle forms.Spindle forms.

Metaphase IIMetaphase II

Chromosomes align Chromosomes align along along equatorequator of cell of cell..

Anaphase IIAnaphase II

Sister Sister chromatidschromatids separate and separate and move to move to opposite polesopposite poles.

EquatorEquator

PolePole

Telophase IITelophase IINuclear envelope Nuclear envelope assembles.assembles.

Chromosomes Chromosomes decondense.decondense.

Spindle Spindle disappears.disappears.

Cytokinesis Cytokinesis divides cell into divides cell into two.two.

Results of MeiosisResults of MeiosisGametes (egg & sperm) Gametes (egg & sperm) formform

Four haploid cells with Four haploid cells with one copy of each one copy of each chromosome chromosome

One allele of each geneOne allele of each gene

Different combinations Different combinations of alleles for different of alleles for different genes along the genes along the chromosomechromosome

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

•This would occur in Anaphase I, when homologous chromosomes are suppose to separate from each other.

Nondisjunction

When a gamete with an extra chromosome is fertilized by a normal gamete, the zygote will have an extra

chromosome.

• This condition is called trisomy.

•An example of this in humans is Downs Syndrome

Nondisjunction

Downs Syndrome

• Extra chromosome on 21

• 1 in every 1100 live births

• Range from mild to severe physical and mental disabilities

• Although organisms with extra chromosomes often survive, organisms lacking one or more chromosomes usually do not.

When a gamete with a missing chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a

chromosome.

• This condition is called monosomy. An example of this in humans is Turner’s Syndrome.

Nondisjunction

Turners Syndrome

• Absence of one X chromosome

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

• Physical characteristics

• No mental disability

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)

How does metaphase I of meiosis differ from metaphase of mitosis? (TX Obj 2; 10A)

Question 3

Metaphase I

During metaphase of mitosis, sister chromatids line up on the spindle's equator independent of each other. During metaphase I of meiosis, homologous chromosomes are lined up side by side as tetrads.

Sister chromatids

Centromere