Meiosis

Question 1

Background:

Answer 1

Chromosomes in cells that are not dividing.

Question 2

Chromosomes:

Answer 2

• Exist as single-stranded structures intertwined to form the chromatic network.
• Occur in homologous pairs called a bivalent. One chromosome of each pair comes from the mother and one comes from the father.
• Homologous - chromosomes that are similar in many ways such as position and structure
• Replicate during the interphase of a cell cycle, ie between cell divisions, to form two identical chromatids (sister chromatids). These are joined by a centromere. This takes place before cells divide by either mitosis or meisos.
• Centromere - a specialized constricted region of a choromosome where the two chromatids are held together and to which spindle fibers attach during cell division.
• Only become visible when the chromosomes shorten and thicken as a cell divides.

Question 3

Meiosis

Answer 3

Meiosis is a special type of cell division that takes place in the reproductive organs of plants and animals to produce gametes (sex cells) in animals and spores in plants.
Cells divide by mitosis. Mitosis enables organisms to grow and to replace tissue. In mitosis replication is followed by one division. In meiosis, replication is followed by two divisions.
- Meiosis 1 is a reduction division which results in two cells being formed each with half the number of chromosomes of the parent cell, ie the haploid (n) number.
- Meiosis 2 is a copying division which involves two haploid cells formed, each dividing again by mitosis to form four daughter haploid cells.

Question 4

Meiosis 1: Reduction Division

Answer 4

The first division separates the homologous chromosomes into two intermediate cells.

Question 5

Meiosis 1: Early Prophase 1

Answer 5

As in mitosis, the chromosomes condense, becoming short and fat. They are visible as two chromatids joined by a centromere. From here on the behaviour of the chromosomes in meiosis 1 differ from that in mitosis.

Question 6

Meiosis 1: Late Prophase 1

Answer 6

• The chromosomes of homologuous pairs lie alongside one another in a process called synapsis to form bivalents.
• The centrioles move to opposite poles.
• A spindle, made up of protein threads, develops across the cell to form the two centrioles. Centrioles are minute rod-shaped bodies formed from the centrosome.
• The nuclear membrane breaks down.
• Parts of the chromatids can become entangled and exchange segments of chorosomes in a process called crossing over.

Question 7

Meiosis 1: Metaphase 1

Answer 7

• the bivalents (not single chromosomes) move to the middle of the cell and line up on the equator in a way that is completely random.
• The centromeres become attached to the spindle treads.
• The centromeres do not split.

Question 8

Meiosis 1: Anaphase 1

Answer 8

• The bivalents separate and the homologues are pulled away from each other by contracting spindle threads. This results in independent assortment of maternal and paternal chromosomes. One chromosome of each pair moves to the opposite pole.

Question 9

Meiosis 1: Telophase 1

Answer 9

The cytoplasm then divides (cytokineses) to form two haploid cells, both of which have only one of each homologous pair of chromosomes.

Question 10

Meiosis 2: A Copying Division

Answer 10

Each of the haploid cells will divide again by mitosis so that the chromatids are pulled apart but the number of chromosomes remains the same. This produces a large number of gametes.

Question 11

Meiosis 2: Late Prophase 2

Answer 11

Each chromosome is made up of two chromatids joined by a centromere. The centriole divides, forming two centrioles. A spindle develops between the centrioles which move to opposite poles. The nuclear membranes dissapears.

Question 12

Meiosis 2: Metaphase 2

Answer 12

The chromosomes move to the middle of the cell where they line up at the equator. The centromeres become linked to the spindle threads.

Question 13

Meiosis 2: Anaphase 2

Answer 13

The centromeres split, allowing each chromosome to separate into two chromatids. Spindle threads contract and pull the chromatids apart. The chromatids, which are now called daughter chromosomes, move to opposite ends (poles) of the cell.

Question 14

Meiosis 2: Telophase 2

Answer 14

Daughter chromosomes group together at the poles. A new nuclear membrane starts to form around each set of daughter chromosomes.

Question 15

Meiosis 2: Cytokinesis

Answer 15

The cytoplasm starts to divide forming two new daughter cells, each with the haploid numbers of chromosomes. This process is called cytokineses. A new nucleus forms and the daughter chromosomes form the chromatin network.
At the end of meisos, four new, non-identical, haploid gametes are formed from one parent cell, each with half the original number of chromosomes. The gametes are not identical to the parent cell.

Question 16

Meiosis: To sum up:

Answer 16

• Two chromatids make up a chromosome.
• Two chromosomes make up a homologous pair.
• A pair of homologous chromosomes makes up a bivalent.

Question 17

What is crossing over?

Answer 17

Crossing over is the mutual exchange of pieces of chromosome so that whole groups of genes are swapped between maternal and paternal chromosomes. This takes place in late prophase to early metaphase of meiosis 1.

• This replicated homologous pairs of chromosomes come together in a process called synapsis to form bivalents. They swap pieces of their inner chromatids by breaking and rejoining their DNA while they ate paired up.
• The points of crossing over where the chromatids break are called chiasmata. In this way, some genes from a maternal chromatic change place with some genes from a paternal chromatid, forming a recombinant chromatid. Recombinants will result in offpspring with genotypes unlike either parent. The outer, unchanged chromatids are called parentals.
• Only one charisma is shown by many may occur per pair of chromosomes.

Question 18

Why is crossing over important?

Answer 18

• The exchange of genetic material produces chromatids with a unique combination of genes. This increases genetic variation among individuals in a population. This is why offspring do not look the same.
• During this exchange process, mistakes may occur which lead to mutations. Most mutations are harmful but occasionally they may be beneficial. New genes may be introduced into the genetic code of a species which is important in the process of evolution as it provides the raw material on which natural selection acts.
• Crossing over guarantees the individual characteristics of each one of us.

Question 19

Purposes of Reduction Division

Answer 19

The cells making up the body of an organism are called somatic cells and in most organisms these cells contain two sets of chromosomes, ie they are diploid (2n).
One set is inherited from the mother and the other from the father. In humans, the diploid number is 46 chromosomes.
- Meiosis is called a reduction division as, during the first meiotic division, the number of chromosomes is reduced from two sets (diploid) in the parent cell to one set (haploid) in each of the daughter cells (gametes) formed. In humans the haploid number is 23 chromosomes.
- Gametes or sex cells in diploid organisms, ie most animals and seed-bearing plants, are made by meiosis so that the number of chromosomes is halved. This process is called gametogenesis. In animals the:
- Formation of sperm - spermatogenesis, occurs in the testes
- Formation of egg cells - oogenesis, occurs in the ovaries
In Angiosperms (flowering plants) meiosis takes place in the anthers and ovaries to produce haploid gametes.

Question 20

How do Mosses and Ferns differ?

Answer 20

An exception is found in primitive plants such as mosses and ferns where haploid spores are formed by meiosis in a diploid sporophyte generation. The spores develop into a haploid gametophyte plant. Specialized cells in the male and female organs of this plant produce haploid gametes by mitosis. On fertilization the zygote develops into a diploid sporophyte.

Question 21

Importance of Meiosis

Answer 21

1. Gametes are formed by the process of meiosis.
2. During meiosis the number of chromosomes is halved so that the chromosome number is kept constant from generation to generation.
3. Meiosis results in genetic variation through:
   - Crossing over
   - Random arrangement of chromosomes at the equator of the cell during metaphase.
   The number of chromosomes in a cell is characteristic of an organism, eg humans have 46 chromosomes.

Question 22

Genetic Variation:

Answer 22

In organisms which reproduce sexually every one of the offspring possess a unique combination of genes, ie in sexual reproduction there is always variation.
Variation - the differences which exists between organisms belonging to the same species.
This variation of the offspring is caused by differences in their genetic code as a result of:
- The crossing over of pieces of chromatids, which causes a reshuffling of the genes in the gametes formed.
- The random lining up of maternal and paternal chromosomes on the equator of the cell during metaphase of meiosis. This is called the independent assortment of chromosomes and it results in every egg and every sperm formed containing a mixture of maternal and paternal chromosomes.
A single set of chromosomes in a human sperm or egg could consist of 17 maternal and 6 paternal or 14 paternal and 9 maternal etc.
- The sheer chance as to which particular sperm fertilizes an egg cell during fertilization. The new individual will have a unique combination of genes, different from either of its parents, and different from any other offspring produced by the same parents.
- Mutations, which are sudden and unpredictable changes in the genetic code of an organism.

Question 23

Abnormal Meiosis

Answer 23

Chromosome non-disjunction is a form of abnormal meiosis as the chromosomes fail to separate correctly.

Question 24

The consequences of non-disjunction:

Answer 24

If this occurs during meiosis in the formation of sex chromosomes, an egg cell or sperm an be formed with two X’s, two Ys or with neither an X or Y chromosome.
If this occurs during meiosis in autosomal pairs, fertilisation involving one of these abnormal pairs, fertilisation involving one of these abnormal gametes results in a zygote with either an extra or missing chromosome - a condition called aneuploidy.
Aneuploidy causes various physical and mental characteristics called clinical syndromes.
Polypoidy is a condition in which an organism (mainly plants) has more than two complete sets of chromosomes in a cell.

Question 25

Down Syndrome

Answer 25

Down syndrome is the most common of the human aneuploidies. It occurs in children who are born with an extra copy of chromosome number 21 in their cells, ie (2n + 1) - a condition known as trisomy.

Question 26

Down Syndrome: How does this come about?

Answer 26

During oogenesis (meiosis in the production of an egg), non-disjunction takes place when two number 21 chromosomes do not separate properly during anaphase 1. Both chromosomes go into one daughter cell instead of one into each of the two daughter formed. This results in an egg with two number 21 chromosomes instead of one. - If this egg is fertilised, the zygote will have three 21 chromosomes and a total of 47 chromosomes in each cell instead of 46. - As the new embroyo develops by mitosis, all the cells will have 47 chromosomes.

Question 27

The Characteristics of Down Syndrome:

Answer 27

- Children have varying degrees of mental retardation. The severity of symptoms differs widely from person to person. - They have distinctive flattened facial features with slightly slanting eyes due to folds of skin at the corner of the eyes. - Other physical features include short stubby fingers and toes with the big toes widely spaced from the second toe, a largish head and ears that may develop differently. - These problems are often accompanied by heart defects and other health problems. - Despite their problems, Down syndrome sufferers tend to have a happy, loving nature. Some of the related health problems such as heart defects may be treated through surgery and medication, but there is no cure for this syndrome.

Question 28

How common is Down Syndrome?

Answer 28

Down syndrome is relatively common, affecting 1 in 900 births. The chances of having a baby with Down syndrome increase with the age of the mother. With age the chromosomes seem to have more difficulty in separating. This is known as the maternal age effect.

Question 29

What problems do Down Syndrome children encounter?

Answer 29

Children with Down syndrome tend to grow and develop more slowly than other children. In many countries, Down syndrome children attend mainstream schools and not special institutions. Some may need special classes to help them in areas where they have more trouble learning. Because they are different from other people, they often encounter patronizing attitudes and discrimination. Parents and teachers need to help each child to learn and live a normal, integrated and carefree life.

Question 30

What about Aborting the Foetus?

Answer 30

A new test, taken early in pregnancy, combines a blood test and ultrasound examination. This can identify many foetuses with Down syndrome 11 weeks after conception, allowing expectant mother more than enough time to decide whether to undergo amniocentesis or to abort the foetus. Amniocentesis - A process whereby a small sample of fluid is removed from the amniotic cavity in the uterus. In the follow-up process, the karotype of these cells can be analysed.

Question 31

A Comparison of Mitosis and Meiosis: Similarities

Answer 31

- Both are types of cell division - The DNA of the parent cell is replicated in interphase before cell division starts. - In early prophase the chromosomes become short and fat and are visible as two chromatids joined by a centromere.

Question 32

A Comparison of Mitosis and Meiosis: Where and When the Process Occurs

Answer 32

Mitosis: - Takes place in plants and animals in the development of a zygote to an embryo and then the cells continue to divide by mitosis to form a mature organism. - In plants it takes place in the meristemmatic tissue to bring about growth. - In animals it takes place in many places. Meiosis: - In plants it takes place in the sporangia of the sporophyte generation, in the formation of spores. - In animals it takes place in the reproductive organs, the testes and ovaries, in the formation of gametes.

Question 33

A Comparison of Mitosis and Meiosis: The Purpose of this Process

Answer 33

Mitosis: The purpose of Mitosis is to keep the number of chromosomes in the daughter cells the same as the number of chromosomes in the parent cell. The cell division is involved in: - Development of an adult organism from a single zygote. - Growth and repair of tissues - Regeneration of body parts - Asexual reproduction. Meiosis: The purpose of Meiosis is to halve the number of chromosomes, ie the diploid number of chromosomes in the parent cells is reduced to the haploid number in the gametes or spores formed. This is to prevent chromosome numbers doubling after fertilization in sexual reproduction.

Question 34

A Comparison of Mitosis and Meiosis: Differences in the Process

Answer 34

- Involves one cell division - In prophase: No bivalents formed and no crossing over. - In anaphase: The centromeres split and the chromatids of each chromosome separate and move to opposite poles of the cell. - Two daughter cells are formed with the same number of chromosomes as the parent cell. - Somatic cells are formed that are similar genetically to the parent cell. Meiosis: - Involves two cell divisions - In prophase 1: Bivalents formed and crossing over takes place. - In anaphase: centromeres do not split and the chromosomes of each homologous pair move to opposite poles. - Four daughter cells are formed, each with half the number of chromosomes of the parent cell. - Gametes are formed that are genetically different to each other and to the parent cell.