lecture 10a: meiosis. topics of discussion 1.introduction 2. meiosis vs mitosis 3. b

LECTURE 10A: MEIOSIS

TOPICS OF DISCUSSION1. INTRODUCTION2. MEIOSIS vs MITOSIS

3. B

INTRODUCTIONMeiosis Definition1. Meiosis is the production of gametes, which is a reduction

division which means a diploid gamete produces haploid gametes - from a full complement of chromosomes to half…

2. Two new cells, produced during mitosis, each have a complete set of chromosomes (a diploid cell)

3. Meiosis reduces the number of chromosomes by half and produces two haploid cells from a diploid cell.

4. A gamete is a cell that is involved in reproduction5. Fertilization restores the diploid (two) chromosome number

by combining the haploid gametes to the fertilized zygote (A zygote is a diploid cell that results from the union of two haploid cells).

6. At the end of meiosis gametes are produced, four cells - each with half a set of chromosomes - haploid cells

7. One set of chromosomes from each parent

Gametes• Sex chromosomes (X & Y)• Autosomes (non-sex chromsomes)• Somatic cells have 2 sets of

autosomes• Gametes have one set of sex

chromosomes and one set of autosomes

Gametes are Haploid• Gametes must have half the genetic material

of a normal cell• If the genetic material in the gametes was not

halved, when they combined the zygote would have more genetic material than the parents

• Meiosis is specialized cell division resulting in cells with half the genetic material of the parents

• Gametes have exactly one set of chromosomes, this state is called haploid (1n)

• Regular cells have two sets of chromosomes, this state is called diploid (2n)

How it works...• The chromosomes are duplicated in

the S phase of interphase (just like mitosis)

• In Meiosis there are 2 cell divisions• In mitosis the chromosomes line up

individually, but in meiosis they line up in homologous pairs, next to each other

Ideogram of the human chromosomes

p-arm ( short )

q-arm( long )

<= centromere

autosomes

Sex chromosomes

NOR, ==>Acrocenticchromosomes13, 14, 1521, 22

Xp21

Terminology

A little more basic terminology

• The

INTRODUCTION1. Meiosis is the production of gametes, which is

a reduction division which means a diploid gamete produces haploid gametes - from a full complement of chromosomes to half…

2. Meiosis reduces the number of chromosomes by half and produces two haploid cells from a diploid cell.

3. A gamete is a cell that is involved in reproduction

4. Fertilization restores the diploid (two) chromosome number to the zygote (A zygote is a diploid cell that results from the union of two haploid cells).

INTRODUCTION

5. Chromosomes are paired – one set are maternal chromosomes, the others are paternal

6. Members of the pair are homologous chromosomes

7. There are two divisions in meiosis: meiosis I and meiosis II. Each division may include a prophase, metapahse, anaphase and telophase

8. As in mitosis the chromosomes are duplicated in the S phase of interphase.

9. In meiosis I, the members of each homologous pair of chromosomes are separated into two daughter cells

INTRODUCTION

10. The chromosomes were duplicated prior to meiosis I so that there are two chromotids in each daughter cell.

11. In meiosis II, chromotids separate into individual chromosomes and enter different haploid daughter cells

12. In Prophase I, homologous chromosomes are paired, one of them is from the mother and one from the father. Remember each chromosome is comprised of two chromatids so that there are four chromatids present at this time.

13. The complex of four chromatids is called a tetrad. The fastening together is called the synapsis. In humans there are 23 tetrads (or 92 chromatids). The number of tetrads is equal to the number of haploid chromosomes.

INTRODUCTION

1. All genes located on a particular chromosome are linked and will tend to be inherited together in in linkage groups.

2. Sometimes this doensn’t happen, during synapsis genetic material may be exchanged bwetween hpomologous chromatids by crossing-over. Crossing-over produces new combinations of of genetic materials. Plant

MEIOSIS vs MITOSIS• In mitosis there is one cell division

resulting in two daughters with the identical set of chromosomes as the parent cell.

• In meiosis there are two cell divisions, producing four daughters. Each cell has the haploid number of chromosomes that is one member of each homolgous pair (one set from each parent)

• So that…

MEIOSIS vs MITOSIS

• So that…• Mitosis - diploid somatic cells yield

diploid somatic cells …one cell divides into 2 duplicates of itself

• Meiosis - Diploid gamete producing cells divide into haploid gametes…one cell divides into four cells but they are a mix of the original

• Meiosis has two divisions. Similar interphase as in mitosis.

MEIOSIS vs MITOSIS• Prophase I there are two homologous

pairs, half of each pair is maternal and half is paternal. Each chromosome consists of two chromatids at this time

• Synapsis or the pairing of homologous pairs results in the coming together of four (two maternal and two paternal) chromatids instead of just two. This is called a tetrad.

• The number of tetrads equals the haploid number of chromosomes (half).

• Since humans have 46 chromosomes, there are 23 tetrads and a total of 92 chromatids.

MEIOSIS vs MITOSIS

• Each cell duplicates the haploid (half of chromosomes) which combines with the other haploids into tetrads.

• Instead of one homologous pair there are two. They separate and one of each pair move to ends of the cell and then the cell separates, another interphase occurs and the second series of meiosis begins.

Stages Of Meiosis

Overview

Steps are similar to mitosis• Duplicated chromosomes (chromatids)

are present after interphase• The chromatids become paired• One of each pair is contributed by the

organism’s mother, the other by the father

• Because the pairing is of chromatids (2n) there are actually 4 chromosomes present in what is called a tetrad

Stages Of Meiosis1. Meiosis resembles mitosis except that it

is actually two divisions not one2. These divisions are called Meiosis I and

Meiosis II3. Meiosis I results in haploid cells with

chromosomes made up of two chromatids

4. Meiosis II is essentially mitosis on haploid cells

5. Stages of meiosis resemble mitosis with two critical differences: the first in prophase I and the second in Metaphase I

6. The number of tetrads is the haploid number of chromosomes (humans have 46 chromosomes, so in prophase 1 there are 92 chromatids and 23 tetrads)

7. Combination process is called synapsis8. Sometimes during synapsis, genetic

material is exchanged between chromosomes in process called crossing over

9. All genes located on a particular linkage group and tend to be inherited together except for crossing over

Stages Of Meiosis - Meiosis I

• Prophase I - The beginning phase– DNA which was unraveled and spread

all over the nucleus is condensed and packaged

– Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids)

– Crossing over, in which chromatids within tetrads exchange genetic material, occurs

Stages Of Meiosis - Meiosis I

• Metaphase I - Middle stage– Tetrads line up along the equator of the

cell and sister chromatids orient toward opposite poles

• Anaphase 1 – Tetrads separate and move toward poles

but chromatids still connected by centromere!

– One copy of each chromosome still composed of two chromatids moves to each pole of the cell

Stages Of Meiosis - Meiosis I• Telophase I - End stage

– New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells

– At the end of telophase 1 there are two doubled chromosomes at each end of cell.

Meiosis

Stages Of Meiosis - Meiosis II• Prophase II

– Cells do not typically go into interphase between meiosis I and II, thus chromosomes are already condensed

• Metaphase II– Chromosomes line up at the equator of the two

haploid cells produced in meiosis I– Because homologous pairs line up in Metaphase 1

there is a 50/50 chance of which one of each pair will go to each pole

– In humans with 23 chromosomes any couple would have 223 x 223 chance of different possible children (about 1 in 70 trillion and this doesn’t account for crossing over!)

Stages Of Meiosis - Meiosis II

• Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells. Centromeres split in anaphase 2 and chromatids move to poles

• Telophase II - Cytokinesis and reformation of the nuclear membrane in haploid cells each with one set of chromosomes made of one chromatid. At the end of telophase there is one chromatid at each end (haploid)

Meiosis

InterphaseInterphase

Mother cellStages Of Meiosis:

Meiosis I

Meiosis IIMeiosis II

Prophase I:Tetrad formation/

crossing over


crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I Stages Of Meiosis: Meiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

The products of mitosis are 2 diploid cells with identical chromosomes.

The products of meiosis are 4 haploid cells each with a unique set of chromosomes.

Prophase IIProphase II


crossing over


crossing over

Crossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

Replication

Chromosomes, Chromatids and Centromeres

Centromere

Chromosome arm

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes

In The Beginning Two1. Humans and many other complex multi-

celled organisms incorporate genetic recombination in their reproduction

2. Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction

3. Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops

4. Meiosis is the process of producing sperm and eggs (gametes)

Egg1n

Haploidnucleus

Fertilization Results In A Diploid Zygote

Sperm2nHaploid

nucleus

Sperm2n

Fertilization Results In A Diploid Zygote

Egg1n

Haploidnucleus

Haploidnucleus

From Zygote to Embryo

Zygote2n

Zygote

2n

CleavageFrom Zygote to Embryo

Cleavage


Morula


Meiosis

Meiosis overview• Meiosis I• Prophase of Meiosis I is longer than

mitotic prophase.• In Metaphase I, the two copies of each

replicated chromosome pair at the metaphase plate (a tetrad), unlike mitosis.

• In Anaphase I, each chromosome moves to pole without chromatid separation.

• At the end of Meiosis I, the chromosome # has been reduced, but each chromosome still has two chromatids.

Meiosis overviewMeiosis II• The chromosomes are not

replicated in the interphase between Meiosis I & Meiosis II.

• The chromatids finally separate in Anaphase II.

• The final result is four haploid gametes, each with half the number of chromosomes present in the diploid cells.

Chromosomes in Meiosis(in a diploid)

• Occurs during prophase I

• Chiasma (pl. chiasmata) indicate where chromosomes have exchanged genetic material

Crossing over

Crossing over(Recombination)

Genes in Meiosis• The

Meiosis, continued

• The

Ideogram of the human chromosomes

p-arm ( short )

q-arm( long )

<= centromere

autosomes

Sex chromosomes

NOR, ==>Acrocenticchromosomes13, 14, 1521, 22

Xp21

Terminology

A little more basic terminology

Euploidy addition or loss of complete sets of chromosomes.

Triploidy complete extra set of chromosomes - 69 caused by fertilization of an egg by more than one sperm or an egg that failed to divide

Tetraploidy complete extra diploid set of chromosome - 92

caused by a failure of the first zygotic division

Aneuploidy gain or loss of a single chromosomefailure in meioses (usually)

Monosomies loss of a chromosome – Turner syndrome autosomal monosomies are

lethal sex monosomies survive

Trisomies gain of a chromosome - Down / Tri 13 / Tri 18 / Klinefelter

Forms of Chromosomal Non-Disjunction

MitosisInterphase

• cells not dividing (majority of time)• chromosomes are decondensed - synthesizing products • all the cell's DNA is duplicated – chromatid duplicates

Prophase• chromatin condenses to form chromosomes • spindle apparatus forms • separation of sister chromatids into 2 daughter cells

Metaphase• spindle to the centromeres • chromosomes move to an equatorial plate (metaphase plate) • metaphase chromosomes can be stained and banded

Anaphase• centromeres divide to create 2 chromosomes instead of a pair • the sister chromosomes are drawn to the opposite poles of the cell • exact division of chromosome, 1 diploid complement

of genetic information to each daughter cell

Telophase• nuclear envelopes reassemble and surround each set

of daughter chromosomes - chromosomes decondense

Mitosis in Action

Meiosis in Action

In meiosis, the process is quite similar to mitosis. However, another cell division takes place in which there is no extra DNA replication step. Instead of having a pair of genes (as in a diploid cell), there is only one copy of each gene (a haploid cell). This one copy of genetic information produces gametes of either sperm or eggs. Thus, only one copy of a gene is passed on to each gamete. It is not until the sperm and egg join that there will be two halves of genetic information. This process is the basis for all of Mendel's laws.

http://www.tokyo-med.ac.jp/genet/mfi-e.htm

http://www.tokyo-med.ac.jp/genet/mfi-e.htm

½ to child ½ to child

Looks like mom and dad

The End