1

Mitosis and Meiosis

Traits are controlled by genes

Each individual has thousands of genes and each gene has two copies in an individual.

What are the physical entities that carry the genes during segregation and independent assortment?

What is the biological basis for Mendel’s laws?

green x yellow

yellow

x

yellow

yellow: yellow:yellow:green

2

Genes reside on Chromosomes

Genes reside on chromosomes, Therefore understanding the behavior and inheritance patterns of individual genes requires an understanding of the behavior of inheritance patterns of chromosomes.

The processes of mitosis and meiosis describe the two basic patterns of chromosome behavior in higher eukaryotes

Mitosis: a form of cell division that produces two daughter cells of identical genotypes

Meiosis: a form of cell division in a diploid cell that produces four haploid cells (not identical)

Meiosis only occurs in a small specialized set of cells known as the germ cells.

3

Development

Mitosis 2N ---->4N ----> 2N+2N (somatic cells)

Meiosis 2N ---->4N ---->N+N+N+N (germ cells)

The segregation and assortment of chromosomes in germ cells is important in the transmission of traits

2N |4N | |N

| | |2N

| |4N

| |2N

mitosis

meiosis

4

The Mitotic cell cycle

The mitotic cycle alternates between the replication of each chromosome (S phase) and the segregation of the replicated chromosomes to two daughter nuclei (M phase).

The intervals between these phases are known as gap phases and this divides the cell cycle into four phases M, G1, S and G2. Interphase consists of G1, S, and G2.

5

Digression: Chromosome number

Smallest number: The female of the ant, Myrmecia pilosula, has one pair of chromosomes per cell. Its male has only one chromosome in each cell.

Largest number: The fern Ophioglossum reticulatum has about 630 pairs of chromosomes, or 1260 chromosomes per cell.

Species Haploid number(n)

Human 23Monkey 21Mouse 20Frog 13Fruit fly 4C. Elegans 6Corn 10S. Cerevisiae 16S. Pombe 3

n=2

Haploids are Diploids are Tetraploids are

6

Mitosis

Mitosis is the period in which the chromosomes condense align along the metaphase plate and migrate to opposite poles. In part because this is the most visibly dramatic stage in the cell cycle much research has focused on these mitotic events.

Net result: The creation of two daughter cells with identical

chromosome complements.

7

Replication of DNA

Each DNA mol is a chromatidThe two chromatids attached to the centromere are called sister chromatids

Homologous Chromosomes

99.99% identical

Mitosis

n=22N

n=24N

8

Mitosis

Sister chromatids line up at the metaphase plate.

Sister chromatids separate to opposite poles

n=24N

n=22N

9

Mitosis in haploid and diploid

Replication of DNA

A a

A A

aa

n=12N

n=14N

A

a

A

a

n=12N

Replication of DNA

A

A A

n=11N

n=12N

A A

n=11N

10

Cell cycle and cancer

Currently the cell cycle/mitosis is an intensively investigated area of research. This is primarily due to the fact that:

1. The structural and regulatory components governing the cell cycle are conserved throughout the phyla. That is, the same proteins are used in yeast, flies and humans. 

2. A number of the mutations that produce cancer in humans disrupt the genes involved in regulating cells during the cell cycle.

Example:

Cancers result from uncontrolled and inappropriate MITOTIC division of cells

Cells actually contain a set of genes whose job it is to prevent cells from dividing inappropriately ( these genes are known as tumor suppressor genes, anti-oncogenes, and more poetically as the "guardians of the cell").

One gene known as p53 ensures that the chromosomes have replicated properly before allowing the cells to proceed into mitosis. Recently it has been found that lesions in this gene are one of the most common in all human cancers.

11

Chromosomes

Basic terms and key features of the chromosome:

Telomere: end of chromosomes

Centromere: It is the constricted region where the microtubules attach and help pull the sister chromatids apart during mitosis

Sister chromatids: replicated chromatids in G2. The two sister chromatids are identical to one another. During prophase and metaphase they look like:

Homologue- chromosome pair in a diploid. They are similar but not identical.

Metaphase plate: the region midway between the two spindle poles in which the chromosomes align during metaphase

Haploid (N)- the condition in which each chromosome is present in one copy (found in gametes)

Diploid (2N): the condition in which each chromosome is present twice as members of a homologous pair

A

A

A A a a

12

Karyotype

Description of length, number, morphology.

Karyotype analysis is extremely important in medicine.

Alternations in karyotypes are linked to birth defects and many human cancers.

Metacentric- centromere in the middle

Acrocentric- centromere towards the end

Telocentric- centromere at the end

13

Downs

Downs Male

Staining and specific banding pattern allows you to line up and identify various chromosomes

Down's syndrome results from an individual possessing three copies of chromosome 21 rather than the normal two. It is the most common of all human defects and occurs in 1/200 conceptions and 1/900 births.

Females over 35 years- segregation defect leading to three copies of chromosome 21

Karyotype analyses of fetus indicates whether the child has Downs.Trisomies occur with other chromosomes as well but usually lead to the death of the fetusXXY and XYY individuals are also found.

Normal Male

14

cri-du-chat

The syndrome cri-du-chat is the result of a deletion in the short arm of human chromosome 5

The affected infant is mentally retarded and has abnormal development of the larynx. Consequently, the most characteristic symptom of the disorder is that their cry resembles that of a cat.

sound recording of:

Normal, Cat, cri-du-chat

infant infant

15

Meiosis

Meiosis:

While the mitotic cycle is designed to produce two cells with the identical genotype, the meiotic cycle is designed to produce four cells each with half of the chromosome complement.

Meiosis allows the cell to maintain constant ploidy (following mating) and at the same time to shuffle the genetic deck (in the progeny)

In meiosis:

Diploid cells undergo one round of chromosome replication followed by two divisions thereby reducing ploidy and producing four haploid cells. The two divisions are referred to as Meiosis I and Meiosis II.

N2N -----> 4N----->N

NN

16

Meiosis

Meiosis I:

Interphase I: chromosomes replicate 

Prophase I: chromosomes condense members of a chromosome pair (homologues) physically associate with one another and lie side by side on the metaphase plate. This process is known as synapsis. The paired chromosome physically overlap forming structures known as chiasma. 

Metaphase I: the paired homologous chromosomes, known as bivalents,move to the center of the cell and line up along the metaphase plate. 

Anaphase I: in a process known as disjunction, the members of a homologous pair migrate to opposite poles. This effectively reduces the total number of chromosomes by half and is therefore called a reductional division. 

17

Meiosis

Telophase I: if this stage were equivalent to telophase of mitosis,

the nuclear envelope would reform and DNA synthesis would take

place. This does not occur and the anaphase meiotic products proceed

directly into Prophase II of meiosis 

Net result: Four haploid meiotic products

Meiosis II is analogous to mitosis; chromosomes, rather than homologous pairs align along the metaphase plate and the chromatids separate

18

19

MeiosisI

Chromosomes replicate

Homologous Chromosomes pair locate on metaphase plate at randomThis is Mendels random assortment

OR

20

Random assortment

anaphaseI. Centromeres do not separateThe two sister chromatids go to the same pole

OR

OR

Reductional division

21

MetaphaseIIa

Cell division without intervening replication!!

Similar to mitotic metaphase

25% 25%

The reduced number of chromosomes in each of the two cells align on the metaphase plate (no pairing of homologous occurs), divide to produce four haploid cells.

22

MetaphaseIIb

Cell division without intervening replication!!

Similar to mitotic metaphase

25% 25%

The reduced number of chromosomes in each of the two cells align on the metaphase plate (no pairing of homologous occurs), divide to produce four haploid cells.

23

Meiosis A a

B b

A A aa

bbB B

A A aa

bbB B

A A

B B

aa

bb

A

B

A

B

a

b

a

b

A A

b b

aa

BB

A

b

A

b

a

B

a

B

24

With 23 human

chromosomes, there

is a possible 223 = 8.4

x 106 distinct

gametes.

Little Alberts 1st edition 9-36© Garland Publishing

1st mechanism for genetic diversity

independent assortment of chromosomes

Unlike mitosis, the meiotic products are not genetically identical

The arrangement of paired homologous chromosomes on the plate in METAPHASE-I is RANDOM. Random arrangement leads to independent assortment of genes on different chromosomes

25

Second mechanism of genetic diversity

The paired homologues physically recombine (or crossover with one another).

26

Crossing over

There are two ways of generating variation:

Random assortment of chromosomes (shuffling of maternal and paternal chromosomes).Recombination between homologous chromosomes (crossing-over)in metaphase I.

Homologous chromosomes pair in metaphaseIAt least one crossover occurs per homologous pair

27

Crossing over is the result of a physical exchange

between homologous chromosomes

Cytological studies in maize by Creighton and McClintock (1931) were the first to demonstrate that recombination is the result of a physical exchange between homologous chromosomes

On chromosome 9 in corn there were two markers:

Endosperm composition: Seed color:

28The genetic recombinants were also cytological recombinants. This strongly supported the model that recombination involves a physical exchange between homologous chromosomes

29

Mitosis and meiosis compared:

30

Chromosome theory of inheritance

As you all know genes reside on chromosomes. This basic fact is called the chromosome theory of inheritance. However earlier in this century, the issue of where the units of heredity resided was fiercely debated.

The notion the genes were located on the chromosomes came from the recognition that the behavior of Mendel's particles during meiosis parallels the behavior of chromosomes at meiosis.

1

2

3

Mendel’s Laws of independent assortment imply that genes on the same chromosome are inherited together and genes on different chromosomes are inherited independently.

31

chromosome theory of inheritance

As you all know genes reside on chromosomes. This basic fact is called the chromosome theory of inheritance. However earlier in this century, the issue of where the units of heredity resided was fiercely debated.

The notion that genes were located on chromosomes came from the recognition that the behavior of Mendel's particles during meiosis parallels the behavior of chromosomes during meiosis.

1. Genes are in pairs, so are chromosomes

2. Alleles of a gene segregate equally into gametes, so do the members of a homologous chromosome pair

3. Different genes act independently, so do different chromosomes

Mendel’s Laws of independent assortment imply that genes on the same chromosome are inherited together and genes on different chromosomes are inherited independently.