Meiosis in Animals - Exercise 13Objectives

-Understand meiosis and know where it occurs.-Know why meiosis is so important.-Be able to explain the phases of meiosis I & meiosis II.-Be able to show meiosis using peptides (simulation).-Know at what three phases rearrangement takes place in meiosis.

MEIOSIS• Occurs only in the germ cells.• Process of gamete production.• One (2n, diploid) cell divides to form four

(1n,haploid) gametes.• Divided into two phases analogous to two

divisions without nuclear replication between them.

• Chromosomes replicate once, the cell divides twice.

Gametes• Unlike somatic cells (skin, liver)–Gametes, sperm and egg cells are

haploid cells, containing only one set of chromosomes

• At sexual maturity–The ovaries and testes produce

haploid gametes by meiosis

Most multicellular organism, as well as many unicellularorganism, produce sexually. They utilize specialized SEX cellscalled GAMETES. All other types of cells are collectively calledSOMATIC cells. In most organisms, there are twomorphologically distinct types of gametes the EGG cell, and theSPERM cell.

Meiosis I used in animals to generate (egg and sperm). It isrequired for animals to do sexual reproduction. The mainfunction of meiosis is to reduce the chromosome number inhalf. In many ways meiosis is very similar to mitosis, the stagesare the same in general terms and Meiosis II is identical tomitosis. Chromosome number of nucleus is reduce by one half from diploid to haploid and chromosome gene carried are shuffled or recombined so daughter cell has unique array of genes. 2 round of chromosome separation occurs in gonads. Ovaries and testis.

• Egg cells are produced by females and sperm cells are by males. Part of the process of fertilization involves the fusion of a sperm nucleus with an egg nucleus to form a single cell, the ZYGOTE. The zygote has one nucleus containing the chromosomes from both the sperm cell and the egg cell.

• Zygote is single cell then divides to form a human. When fertilization fuses to form single cell it’s called a zygote, which has one nucleus containing chromosomes of sperm and egg.

Since sexual reproduction involves the combining of chromosomes from twodifferent cells, and since the number of chromosomes per cell remainsconstant within members of a species from generation to generation, theremust be a reduction of chromosomes number in each generation to offsetthe increase at fertilization. In a normal somatic cell of an organism, thereare actually two complete sets of chromosomes. For each chromosome,there is a “, and other morphological characters. These twochromosomes apartner” chromosome which is identical in terms of length,location of kinetochorelso contain genes for the same characteristics, although

notnecessarily for the same form of the characteristic (i.e body size – one genemay specify tall, the other gene may specify short, but both genes specify bodysize). Such chromosomes are said to be HOMOLOGOUS, and one of each of themwas provided by each parent. A cell that has two complete sets of chromosomesis said to be DIPLOID or to contain the 2N number of chromosomes. A cell thathas only one complete set of chromosomes is HAPLOID and contains the Nnumber of chromosomes. In animals, normally somatic cells are diploid, whilenormal gametes are haploid.

OVERVIEW OF MEIOSIS• Each gamete has the potential to be genetically

unique due to the processes of crossing over and independent assortment.

• Independent assortment- Alignment of the tetrads in Metaphase I is independent of each other, this allows the reshuffling of genetic material.

• The probablility that all chromosomes from the same parent assort into the same gamete is equal to (1/2)n. Where n=number of homologous pairs.

• In humans: P=(1/2)23=1/8,368,608.

Significant Events in the stages of Meiosis

The process of meiosis, like that of mitosis, is acontinuous sequence of events that have beenarbitrarily divided into stages for convenience. Sincethere are two rounds of chromosomes separation,The two divisions are called MEIOSIS I and MEIOSISII. Each division stage is further separated intoprophase, metaphase, anaphase, and telophasefollowed by the appropriate Roman numeral (e.g.,prophase I). Prophase I is a very long, complicatedprocess, and it too is subdivided into several substages.

MEIOSIS I

• “Reductional Division”• Number of chromosomes (centromeres) is

reduced by ½.• Homologous chromosomes separate from

each other into separate cells.• One diploid cell (2n) divides to form two

haploid cells (1n).

Homologous Chromosomes• Chromosomes that pair up during meiosis– Contain the same genes– May have different alleles of these genes– One came from each parent

• Each is one long DNA molecule– A gene is a short region of the molecule– Each chromosome can have > 1,000 genes

The process of MEIOSIS is modified version of the mitoticcell division process. During meiosis, two highlysignificant consequences occur: (a) the chromosomenumber of the nucleus is reduced by one half from thediploid (2N) to the haploid (N) number, and (b) thechromosomes (and thus the genes that they carry) areshuffled or recombined so that each daughter cell has aunique array of genes. It is this latter effect that causesno individuals (except identical twins) to be exactly alike.As in mitosis, meiosis is preceded in the cell cycle bychromosome duplication; however, in meiosis there aretwo rounds of chromosomes separation or two celldivisions called meiosis I and meiosis II. In animals, themeiotic process occurs in the gonads. The gonads arecalled the OVARIES in females and TESTES in males.

PROPHASE I

• Homologous chromosomes find each other and associate along their entire length-SYNAPSIS.

• Synapsis causes the Tetrad stage of meiosis to form where each homologous pair consists of 4 chromatids in close association

• Non sister chromatids in the tetrad undergo crossing over and exchange pieces.

• Chiasmata are the areas where the chromatids cross over.

Meioses IProphase I:• The chromosomes are duplicated during the S stage of the cell cycle, before

meiosis begins, and each of them exists a pair of chromatids joined at their kinetochores. During prophase I the homologous pairs of chromosomes come together and align themselves precisely, from end to end, gene for gene. This process is called SYNAPSIS. Because each member of the homologous pair is composed of two chromatids, there is a total of four strands (chromatids) in a synapsed pair. This set is called a TETRAD. There will be as many tetrads in the cell as there are chromosomes in haploid set (e.g., 23 tetrads in human cells). As prophase I proceeds, the chromatids in a tetrad become twisted around one another. Occasionally, two homologous chromatids break in the same relative location and exchange places with one another. This is called CROSSING OVER, and it produces two recombinant chromatids. These recombinant chromatids have some genes from one chromosome and some from another (a mix of some maternal and some paternal genes). This is the first way in which new gene combination may be produced during meiosis.Homeolgous chromosomes associate and crossing over can occur.

METAPHASE I

• The homologous chromosomes (tetrads) line up on the center line of the cell.

• Differs from mitosis in that centromeres do not duplicate.

• Independent assortment occurs here.

Meioses I• Metaphase I

• The tetrads remain intact throughout prophase I, and the chromosomes attached to spindle fibers and migrate to the equator of the cell (metaphase plate) as tetrads. Spindle attachment is random – i.e., which chromosome of a tetrad is facing which pole is determine purely by chance.

ANAPHASE I

• Maternal (mom) and paternal (dad) homologues of the tetrad separate and go to opposite poles of the cell.

• This process is known as disjunction.

Meioses I• Anaphase I

• During anaphase I, homolgous chromosomes are pulled away from one another intact (the chromatids are not separated). Thus one homolog form a tetrad will go to one pole, and the other homolog will go to the other pole. Only one half of the total number of chromosomes will go to each pole. The chromosomes number will have been reduced from the diploid to the haploid count in the resultant nuclei. Meiosis I is, therefore, called the REDUCTION DIVISION. Because the spindle attachment is random, the tetrads will separate independently of one another. This is called INDEPENDENT ASSORTMENT, and it is the second way in which meiosis produces genetic recombination.

TELOPHASE I

• Maternal and paternal homologues reach opposite poles of the cell.

• Two haploid cells are formed which are not identical.

Meioses I• Telophase I

• Once the chromosomes reach the poles and telophase I and cytokinesis I have been completed, the two daughter cells are haploid, but the chromosomes are still composed of two chromatids stuck at their kinetochores. Without any further chromosomal duplication, the two cells will go through a second stage of division called Meiosis II.

• 2 cells are formed that ARE NOT identical !!!

MEIOSIS II

• “Equational Division”• Number of chromosomes (centromeres) is

equal before and after the division.• Sister chromatids separate from each other

into separate cells.• Two haploid cells (1n) divide to form four

haploid gametes (1n).

MEIOSIS II

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

PROPHASE II

• May or may not occur, no significant events here.

METAPHASE II• Chromosomes arrange themselves on the

center line of the cells.• Centromeres duplicate.

ANAPHASE II

• Sister chromatids separate to opposite poles of the cells.

TELOPHASE II• Chromatids reach the opposite poles of the

cells. Nuclear membrane reforms etc.• Four haploid (1n) gametes result.

Meioses II

• Meiosis is mechanically just like mitosis. The individual chromatids are separated at anaphase II. Meiosis II is called EQUATIONAL DIVISION. Once meiosis II and cytokinesis II are completed, there will be four haploid daughter cells. Each cell will have one complete set of chromosomes, and each cell will be genetically unique because of crossing over and independent assortment. The third mechanism of recombining genes occurs during fertilization when two gametes fuse at random to form a zygote.

• The significance of meiosis and sexual production lies in the production of genetic variability within the species. The variety affords the species as a whole a wilder opportunity to successfully meet new environmental challenges to its survival. The genetic variety is introduced by (1) crossing over; (2) independent assortment; and (3) random union of gametes.

Synapsis and crossing over–Homologous chromosomes physically

connect and exchange genetic informationTetrads n the metaphase plate–At metaphase I of meiosis, paired

homologous chromosomes (tetrads) are positioned on the metaphase plates

Separation of homologues–At anaphase I of meiosis, homologous

pairs move toward opposite poles of the cell–In anaphase II of meiosis, the sister

chromatids separate

Summary of Meioses II• Prophase II: NO crossing can occur. • Telophase II: -Each new cell has ½ the amount of DNA as the original parentcell.-Each has only one copy of each chromosomes (haploid)-These cells will become either egg or sperm cells. • In males, gamete formation will result in 1 parent cell producing 4

viable sperm cells and in females, it will result in 1 parent cell producing one 1 viable egg cell , which both are haploid cells.

• No interphase in meiosis, it goes straight to prophase II (probably

didn’t condense much because didn’t have an interphase stage) – people say there’s not much difference between telephase I and Prophase II.

Main difference between Meiosis I & Meiosis II

• Meiosis I- homologous chromosomes separate.

• Meiosis II- sister chromatids separate.

• Homologous chromosomes- are genetically equivalent but not identical. One inherited from mother, one from father.

• Three events are unique to meiosis, and all three occur in meiosis l:

– Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poles of the cell

Genetic Variation• Independent assortment of chromosomes– Each pair of homologous maternal and paternal

chromosomes are arranged independently of the other pairs allowing for many possible combinations chromosomes (2n, where n = haploid number)• Humans = 8 million possible gametes

• Crossing over– Homologous portions of two nonsister chromatids trade

places (2-3 crossovers/chromosome)– Recombinant chromosomes are produced which combine

genes from both parents• Random fertilization– The combination of egg (8 million combinations) and

sperm (8 million combinations) produce a zygote with any of about 64 trillion diploid combinations

Three mechanisms contribute to genetic variation:

1. Independent assortment ofchromosomes

2. Crossing Over

3. Random fertilization

INDEPENDENT ASSORTMENT

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis.

• The purpose of meiosis I is to separated the maternal and paternal homologues.

• The number of combinations possible is 2n, where n is the haploid number.

• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes.

CROSSING OVER

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.

• Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene.

• In crossing over, homologous portions of two nonsister chromatids trade places.

• Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.

Crossing Over

• Homologous chromosomes line up during meiosis• Parts of maternal and paternal chromosomes migrate

Before Meiosis

Meiosis

Gametes produced

RANDOM FERTILIZATION

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg).

• The fusion of human gametes produces a zygote with any of about 70 trillion diploid combinations.

• Crossing over adds even more variation.• Each zygote has a unique genetic identity.

Simulation of Meiosis • The simulation of meiosis, using pipe cleaners as chromosomes.

• The simulation of meiosis will illustrate all of the major events of meiosis except crossing over.

• The simulation uses different colors and different sizes of pipe

cleaners to represent chromosomes.

• If the pipe cleaners (chromosomes) are the same size, they are defined as homologous.

• If they (chromosomes) are the same size and the same color, they are identical and therefore, sister chromatids.

• The male and female snaps on the pipe cleaners represent the kinetochore (centromere).

• The sheet of paper represents the cell. The simulation is for an imaginary species of animal that has an N number of 2 and a 2N number of 4 (2 homologous pairs).

• Spermatogenesis will be simulated, not oogenesis. Therefore, the results of this simulation will be the formation of four haploid sperm cells. As you go through the simulation, try to relate the behavior of the pipe cleaners (chromosomes) to the ultimate consequences of meiosis (the reduction of the chromosome number and genetic recombination).

A COMPARISON OF MITOSIS & MEIOSIS

• Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell.

• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell.

• The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis.

Reshuffling of genetic material in meiosis-Produces genetic variation-Essential to evolution

• Natural selection results in accumulation of genetic variations favored by the environment.

• Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations.

EVOLUTIONARY SIGNIFICANCE OF GENETIC VARIATION (MEIOSIS)

Summary

• Events unique to meiosis– Synapsis: homologous chromosomes pair– Chiasmata: crossing over of nonsister chromatids

to exchange genetic information– Meiosis I separates homologous chromosomes• Meiosis II is physically identical to mitosis

Diploid cell with duplicated

chromosomes

2 genetically identical diploid cellsMITOSIS

MEIOSIS4 genetically different haploid cells

A

A

a

a

A

A

a

a

A

A

a

a

NORMAL DISJUNCTION

Tetrad Stageof Meiosis

1st division

Meiosis I

2nd division

Meiosis II

Gametes

N (haploid)

N (haploid)

N (haploid)

N (haploid)

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

Chromosomal Life Cycle Animal Life Cycle• In animals– Meiosis occurs during

gamete formation– Gametes are the only

haploid cells

Gametes

Diploidmulticellular

organism

Key

MEIOSIS FERTILIZATION

n

n

n

2n2nZygote

HaploidDiploid

Mitosis

(a) Animals

Human Life Cycle

46 23Meiosi

s

Sperm cell

4623

Egg cell

Meiosis

46

Zygote

Mitosis

Somatic cell

Somatic cell

Page 6 – Lab Book

• 1. Distinguish between the haploid and diploid chromosome numbers of a species.

• 3.

a) Which meiotic division is the reduction division?

b) Which meiotic division is the equationaldivision?

Page 6 – Lab Book

• 1. Distinguish between the haploid and diploid chromosome numbers of a species.

• The diploid condition has homologous pairs. • Haploid = 23 (half) (n)• Diploid = 46 (all) (2n) • 3.

• a) Which meiotic division is the reduction division? Meiosis I• b) Which meiotic division is the equational division? Meiosis II

Page 6 – Lab Book

• 4. What is the difference between a reduction division and an equational division in terms of chromosome number?’

• 6.

a) During which stage of meiosis does synapsis occur? b) Does synapsis occur during mitosis? If so, when? c) Why is it important for synapsis to occur in meiosisII?

Page 6 – Lab Book• 4. What is the difference between a reduction division and an equational

division in terms of chromosome number?’

• Reduction division reduces the chromosomes number by half. • The equational division the same chromosome number as the original

parent cell. • 6.

a) During which stage of meiosis does synapsis occur? Prophase Ib) Does synapsis occur during mitosis? If so, when? No c) Why is it important for synapsis to occur in meiosis II? There areno homolgous pairs. The cells going through meiosis II are haploid. There’s no homologus pairs to line up.

Page 6 – Lab Book• 8. Describe a tetrad. • 9. During what stage of meiosis does crossing over occur? • 10. During what stage of meiosis does independent assortment

occur? • 14. List 3 ways by which meiosis and/or sexual reproduction can

increase genetic variability in a species.

a)b)c)

Page 6 – Lab Book• 8. Describe a tetrad.-A synapsed homologous pair of chromosomes-4 sister chromtides of a particular n chromosome. • 9. During what stage of meiosis does crossing over occur? Prophase I • 10. During what stage of meiosis does independent assortment occur?

Anaphase I • 14. List 3 ways by which meiosis and/or sexual reproduction can increase

genetic variability in a species.a) Crossing overb) Independent assortmentc) Union of different gametes (unique fertilization)