6.2 process of meiosis how many chromosomes? normal human body cells each contain 46 chromosomes....

6.2 Process of Meiosis

How Many Chromosomes?

• Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. Working with a partner, discuss and answer the questions that follow.

Section 11-4Interest Grabber

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• 1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis?

• 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? (Shrimp)

• 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

Section 11-4Interest Grabber continued

Go to Section:


• 11–4 MeiosisA. Chromosome Number

B. Phases of Meiosis

1. Meiosis I

2. Meiosis II

C. Gamete Formation

D. Comparing Mitosis and Meiosis

Section 11-4

Section Outline

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I.Chromosomes

Chromosomes- condensed DNA

before division = two copies (chromatids) of the same genetic code


A.Chromosome Number

1.Different species have different numbers of chromosomes.

– Fruit flies =8, dogs = 78, chimps = 48

2. Chromosomes paired because one set from each parent.

3. Human cells (except sex cells) have 23 pairs = 46 total chromosomes: 23 from mom and 23 from dad


B. HOMOLOGOUS Chromosomes

Chromosome pairs (1 from each parent) that code for the same type of genes

but not necessarily the same expression.

EX: a pair of homologous chromosomes may have genes for hair color, but the one from mom codes for brown hair and the one from dad codes for red.

We all have two copies of our genetic code.

6.2 Process of Meiosis Homologous Pairs

In all offspring

6.2 Process of Meiosis HOMOLOGOUS Chromosomes


CheckpointHomologous Shoes

1. Observe the shoes on the next slide. What do you notice? How are pairs of shoes similar to pairs of chromosomes?

2. Now compare within the pairs. What do you notice?

9ANY QUESTIONS?


Where are genes located in the cell?•On the chromosomes.

•The genes segregate during gamete formation so that each will have just one full set of instructions.

•What is diploid? (2N)•Literally two sets, it is a cell that contains two sets of chromosomes.


KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.


C. DIPLOID v HAPLOID

Diploid (2n) = “two sets” - a cell with paired homologous chromosomes

most cells have a set from each parent

• Human diploid (2n) cells = 4623 from each parent

n = #chromosomes from each parent

2n = total # of chromosomes


C. DIPLOID v HAPLOID

2. Haploid (n) - a cell that contains only one set of the chromosomes

– Human haploid cells (n) = 23

– Sex cells or GAMETES


D. Mitosis Chromosomes

What was the purpose of mitosis?

– To divide into two cells that are Identical to original cell so same chromosome number and exact genes

– Diploid cells undergo mitosis to produce diploid daughter cells

– If one cell in a dog with 2n=78 undergoes mitosis, how many chromosomes will new cell have?


92

If a new human is created from the union of two cells (one from the father and mother)…

• How then, do you not double your chromosome number each time?

46

46

egg(mother)

sperm(father)

zygote(cell)


• If sex cells (gametes) were diploid, each generation would double its chromosome number.

• Two gametes with 46 would produce a zygote with 92 chromosomes.

• In the next generation, 92 + 92 = 184 chromosomes…and in the next generation we would have 368, and…..

9246 46

zygote(cell)


Obviously, this can’t happen

• Cell would have too many instructions

• Nucleus would burst with too many DNA molecules– A single DNA strand is about 5 cm (2 in) long– Our 46 DNA molecules paled end to end

would be about 2 meters long

Why is this a problem?


A.Sex cells are special.

• The cells used to pass on genes from parent to offspring are different than every other cell in your body.

• GAMETES - haploid sex cellsSperm(male) and egg (female)

Have half the normal number of chromosomes (hapliod)

produced by meiosis, not mitosis.


Checkpoint

1. Are cells from meiosis the same or different than the parent cell?

2. How are they different?3. Why is this important?4. What type of cells undergo meiosis? 5. If one cell in a dog with 2n=78 undergoes

meiosis, how many chromosomes will new cell have?

20ANY QUESTIONS?

Meiosis

challenge


• What is Meiosis?• The process where diploid cells form haploid

cells. Reduction division separates the chromosome pairs in a diploid cell.


Cells go through two rounds of division in meiosis.

• Meiosis reduces chromosome number and creates genetic diversity.


• Meiosis I and meiosis II each have four phases, similar to those in mitosis.

homologous chromosomes

sisterchromatids

sisterchromatids

– Pairs of homologous chromosomes separate inmeiosis I.

– Homologous chromosomes are similar but not identical.– Sister chromatids divide in meiosis II.– Sister chromatids are copies of the same chromosome.


• Meiosis I occurs after DNA has been replicated.• Meiosis I divides homologous chromosomes in four

phases


Meiosis

A. 2 STAGES:

• Meiosis I – separates homologous chromosomes to form two new haploid cells

6.2 Process of Meiosis Meiosis

A. 2 STAGES:• Meiosis I – separates

homologous chromosomes to form two new haploid cells

• Meiosis II – separates chromatids in the two cells from first stage to form four haploid cells


B.Phases of Meiosis I

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase I


Phases of Meiosis I: Interphase I

DNA replicates – makes sister chromatids.


Phases of Meiosis I: Prophase I

• Tetrad - paired homologous chromosomes = 4 chromatids

a) Spindles formb) Nucleus disappearsc) Chromosomes condensed) Homologous

chromosomes pair up to make a TETRAD

e) Crossing Over occurs.


CROSSING OVER

Homologous chromosomes (tetrads) exchange portions of their chromatids during prophase I = new combinations of GENES.


Phases of Meiosis I: Metaphase I

Tetrads line up with homologues on each side of the line (NOT IDENTICAL)

Tetrads line up in random order = genetic variations

Crossed over sections


Meiosis I = New Combinations• Two different ways to line up with 2 pairs = 22 or 4

combinations.• With our 23 pairs = 223 or 8,388,608 combinations!


Phases of Meiosis I: Anaphase I

Spindles pull the homologous chromosomes apart (separates tetrads)


Phases of Meiosis I: Telophase I and Cytokinesis

• Nuclear membranes reform.

• Cytokinesis = makes 2 cells

New cells are haploid and genetically

different than parent cell or each other.


• Meiosis II divides sister chromatids in four phases.• DNA is not replicated between meiosis I and

meiosis II.


Phases of Meiosis II

Telophase II and

Cytokinesis

Prophase II Metaphase II Anaphase II

Meiosis I

Meiosis II

Second stage of meiosis is exactly like mitosis, but with haploid cells.Chromosomes line up, chromatids travel to opposite sides of the cell, and the cell splits.


Phases of Meiosis II

Telophase II and

Cytokinesis

Prophase II Metaphase II Anaphase II

Meiosis I

Meiosis II

Both cells from Meiosis I go through Meiosis II = Makes 4 haploid cells with different combinations of genes on the chromosomes.


• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.– In mitosis, homologous chromosomes never pair up.– Meiosis results in haploid cells; mitosis results in diploid

cells.


Haploid cells develop into mature gametes.

• Gametogenesis is the production of gametes.

• Gametogenesis differs between females and males.

– Sperm become streamlined and motile.

– Sperm primarily contribute DNA to an embryo.

– Eggs contribute DNA, cytoplasm, and organelles to an embryo.

– During meiosis, the egg gets most of the contents; the other cells form polar bodies.


How can siblings look alike but not exactly the same if they come from the same parents with the same DNA?


Because not all of the cells produced in meiosis are identical.

Meiosis produces new combinations of your parents genes!

The gene combinations that a person gets from his or her parents will be different, to varying degrees, than the combination a sibling may get.


Checkpoint

1. What is crossing over and when does it occur?

2. How do you get new genetic combinations in metaphase I?

3. Are cells haploid or diploid at the end of meiosis I?

4. What are male gametes called and how many are made during meiosis?

5. Why is only one egg made during female meiosis?

42ANY QUESTIONS?


Forever Linked?

• Some genes appear to be inherited together, or “linked.” If two genes

• are found on the same chromosome, does it mean they are linked forever?

• Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.

Section 11-5Interest Grabber

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• 1. In how many places can crossing over result in genes A and b being on the same chromosome?

• 2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?

• 3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

Section 11-5Interest Grabber continued

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13.1 Ecologists Study Relationships

Section 11-4Crossing-Over

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• 11–5 Linkage and Gene MapsA. Gene Linkage

B. Gene Maps

Section 11-5Section Outline

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KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.


VI. Sexual Reproduction

Fertilization - union of two haploid gametes to form a unique diploid zygote

• Two parents = offspring with genetic diversityMore variation because random which sperm

will fertilize which egg : 223 x 223 = 70 trillion combos

Zygote = fertilized egg

Humans - sperm(23 chromosomes) + egg (23 chromosomes) = zygote with 46 chromosomes.


N=23N=23

2N=46


VII. Determining your gender

Human Karyotype

Autosomes - 22 pairs of our chromosomes have the same type of information

Sex chromosomes – differ from male to femaleX X= femaleXY = male


IX. MEIOSIS – The PURPOSE

•Meiosis creates haploid gametes with new gene combinations for sexual reproduction = increased genetic diversity of offspring.

How?1.DNA gets mixed up during crossing over of prophase I2.DNA gets mixed up during line up in metaphase I3.Produces haploid cells so don’t constantly double DNA each generation4.DNA gets combined from two different parents during fertilization (sexual reproduction).


Sexual reproduction creates unique combinations of genes.

• Sexual reproduction creates unique combination of genes.– independent assortment of chromosomes in meiosis– random fertilization of gametes

• Unique phenotypes may give a reproductive advantage to some organisms.


Crossing over during meiosis increases genetic diversity. • Crossing over is the exchange of chromosome

segments between homologous chromosomes.– occurs during prophase I of meiosis I– results in new combinations of genes


• Chromosomes contain many genes.– The farther apart two genes are located on a

chromosome, the more likely they are to be separated by crossing over.

– Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.

• Genetic linkage allows the distance between two genes to be calculated.


• What is gene linkage?

• A group of genes that are inherited together.

• Each chromosome is a group of linked genes.

• These groupings of genes, or chromosomes will assort independently


• Linked genes can be transferred to other chromosomes during cross over.

• The rate at which crossing over occurs can be used to map the gene location on the chromosome.


Earth

Country

State

City

People

Cell

Chromosome

Chromosome fragment

Gene

Nucleotide base pairs

Section 11-5Comparative Scale of a Gene Map

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Mapping of Earth’s Features

Mapping of Cells, Chromosomes, and Genes


Exact location on chromosomes Chromosome 2

Section 11-5 Figure 11-19 Gene Map of the Fruit Fly

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Why is diversity important?

1. Helps a population survive environmental change– Ex: I am legend– slight gene variations are necessary to produce

changes in anatomy that enable organisms to adapt and survive. More diversity = more variations from which to choose

2. Helps a population survive unfavorable mutation– If no variation, then every offspring would have the

mutation.– If lethal, all genes from that line would be wiped out.

3. Evolution depends on genetic diversity.1.No change in species without variation.2.Ex: wisdom teeth,


Meiosis + Fertilization + Mitosis = YOU

1. Meiosis = sperm and egg from your parents

2. Fertilization (sex!)– joining of egg and sperm formed single cell zygote

3. Mitosis = zygote cell divides into trillions of cells or baby you.

4. Mitosis continues in you for growth, development and repair.

5. Meiosis occurs in your ovaries or testes so you can make gametes for sex too!


Checkpoint

1. Is it the parent cell that makes gametes haploid or diploid? Would it undergo mitosis or meiosis?

2. In what ways does meiosis increase genetic diversity in a population?

3. What is a zygote? (How is it formed?)4. To grow into an adult, would cells undergo

mitosis or meiosis?

63ANY QUESTIONS?

6.2 process of meiosis how many chromosomes? normal human body cells each contain 46 chromosomes....

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