Mitosis And Meiosis

Question 1

How many DNA molecules is one chromosome?

Answer 1

Can be 1

Can be 2 (when 2 sister chromatids joined together - this is also called a chromosome)

Question 2

Describe the structure of a chromosome

Answer 2

The classical X-shape is a replicates chromosome containing 2 identical molecules (sister chromatids)

P arm - short arm
Q arm - long arm
Telomere - ends
Centromere - centre - region which microtubules attach to

Question 3

What are telomeres?

Answer 3

Repeated sequences TTAGGG at the end of each chromosome

Question 4

What are centromeres?

Answer 4

Link the sister chromatids and consist of repetitive sequences - also where the spindle fibres attach

Question 5

How are chromosomes grouped?

Answer 5

According to shape and size
Groups A-G
X is in C group
Y is in g group

Question 6

What is mitosis

Answer 6

Cell division for somatic cells
Production of 2 identical daughter cells
~50 mitosis rounds during development
Mitosis growth necessary for some tissue (epidermis, mucosae, bone barrow, spermatogonia)

Question 7

What are the stages in the cell cycle?

Answer 7

G1
S
G2
M (prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis)

Question 8

Describe prophase

Answer 8

In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes.
The chromosomes start to condense (making them easier to pull apart later on).
The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart.
The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down.

Question 9

Describe prometaphase

Answer 9

In late prophase (sometimes also called prometaphase), the mitotic spindle begins to capture and organize the chromosomes.
The chromosomes finish condensing, so they are very compact.
The nuclear envelope breaks down, releasing the chromosomes.
The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.
Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.)
Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that don’t bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster.

Question 10

Describe metaphase

Answer 10

In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide.
All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up).
At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles.
Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed.

Question 11

Describe anaphase

Answer 11

In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell.
The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell.
Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer.
All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.

Question 12

Describe telophase

Answer 12

In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place.
The mitotic spindle is broken down into its building blocks.
Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear.
The chromosomes begin to decondense and return to their “stringy” form.

Question 13

Describe cytokinesis

Answer 13

Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.
In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.

Question 14

What are homologous chromosomes?

Answer 14

• homologous chromosomes have the same DNA sequence, the same genes
• genes on homologous chromosomes may be different variants (alleles)

Question 15

What are sister chromatids?

Answer 15

• sister chromatids have identical DNA, the same alleles

* non-sister chromatids have the same genes, but maybe different alleles

Question 16

What is meiosis?

Answer 16

• special cell division for germ line cells
• production of four non-identical cells (with half chromosome content of parental cell)
• one round of replication followed by two rounds of division (meiosis I and II)
• diploid (2n) reduced to haploid (n)
• production of eggs and sperm

Question 17

What happens in prophase 1?

Answer 17

During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length.
Crossing over occurs

Question 18

What are chiasmata?

Answer 18

You can see crossovers under a microscope as chiasmata, cross-shaped structures where homologues are linked together. Chiasmata keep the homologues connected to each other after the synaptonemal complex breaks down, so each homologous pair needs at least one. It’s common for multiple crossovers (up to 25!) to take place for each homologue pair

Question 19

What happens in metaphase 1?

Answer 19

After crossing over, the spindle begins to capture chromosomes and move them towards the center of the cell (metaphase plate). This may seem familiar from mitosis, but there is a twist. Each chromosome attaches to microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles. So, during metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation.
The orientation of the homologous pairs is random - independent assortment - this creates variation

Question 20

What happens in anaphase 1?

Answer 20

In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don’t come apart.

Question 21

What happens in telophase 1?

Answer 21

Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes decondense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II
Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells.

Question 22

What happens in prophase II?

Answer 22

During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.

Question 23

What happens in metaphase II?

Answer 23

The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate.

Question 24

What happens in anaphase II?

Answer 24

In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell.

Question 25

What happens in telophase II?

Answer 25

In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.

Question 26

What are the consequences of meiosis?

Answer 26

• maintaining constant chromosome
number of from generation to generation
• generation of genetic diversity
1. random assortment of chromosomes 
2. crossing over of genetic material

Question 27

How many spermatozoa (n) are made from 1 spermatocyte (2n)?

Answer 27

4

Question 28

How many mature ova (n) are made from 1 oogonium (2n)

Answer 28

1 ovum + 3 polar bodies