2. Cells - Mitosis, Meiosis And Binary Fission

Question 1

How many daughter cells does mitosis produce?

Answer 1

2

Question 2

How many chromosomes does the daughter cells produced by mitosis have?

Answer 2

They have the same number of chromosomes as the parent cell and each other.

Question 3

What is the first stage before cell division in mitosis and meiosis?

Answer 3

Interphase.

Question 4

What are the phases of interphase?

Answer 4

• G1 phase
• S phase
• G2 phase

Question 5

What happens during G1 of interphase?

Answer 5

• The cell grows in size
• New organelles are produced
• Normal cellular functions and activities occur

Question 6

What happens during S phase of interphase?

Answer 6

• DNA replication takes place
• Each chromosome is replicated to form two indentical sister chromatids joined at the centromere

Question 7

What happens during G2 of interphase?

Answer 7

• The cell continues to grow
• It prepares for mitosis by producing proteins and organelles necessary for cell division

Question 8

What is the second stage of mitosis?

Answer 8

Prophase.

Question 9

What happens during prophase?

Answer 9

• Chromosomes first become visible, then later condense (shorten and thicken)
• Animal cells contain organelles called centrioles, each to which move to opposite poles of the cell
• Spindle fibres develop from each centriole
• The nucleolus disappears
• The nuclear envelope breaks down, leaving the chromosomes free in cytoplasm of cell
• Chromosomes are drawn towards the equator of the cell by spindle fibres attached to centromere

Question 10

What do plant cells, for example, that don’t contain centrioles, have the equivalent of?

Answer 10

They develop a spindle apparatus (a collection of spindle fibres)

Question 11

What is the third stage of mitosis?

Answer 11

Metaphase.

Question 12

What happens during metaphase?

Answer 12

• The chromosomes are seen at this stage to be made up of two chromatids
• Each chromatid is an identical copy of DNA from the parent cell
• Chromatids are joined by a centromere, it is this that some microtubules from the poles are attached, and the chromosomes are pulled along the spindle apparatus and they then arrange themselves across the equator of the cell

Question 13

What is the fourth stage of mitosis?

Answer 13

Anaphase.

Question 14

What happens during anaphase?

Answer 14

• The centromeres divide into two and the spindle fibres pull the individual chromatids apart
• The chromatids move rapidly to their opposite poles of the cell and we now refer to them as chromosomes

Question 15

What is the fifth stage of mitosis?

Answer 15

Telophase and cytokinesis

Question 16

What happens during telophase?

Answer 16

• The chromosomes have reached their respective poles
• and become longer and thinner, finally disappearing altogether, leaving only widely spread chromatin
• The spindle fibres disintegrate
• The nuclear envelopes and nucleolus re-form

Question 17

What happens during cytokinesis?

Answer 17

The cytoplasm divides during this phase.

Question 18

How many daughter cells does meiosis produce?

Answer 18

4

Question 19

How many chromosomes does each daughter cell produced by meiosis have?

Answer 19

Half the number of chromosomes as the parent cell

Question 20

What happens during the first division of meiosis? (Meiosis 1)

Answer 20

• Each chromosome lines up alongside its homologous partner and their chromatids wrap around each other
• Equivalent portions of these chromatids may be exchanged in a process called crossing over
• Independent segregation - since the pairs line up at random, the combination of chromosomes that go into the daughter cell is a matter of chance
• By the end of this division the homologous pairs have separated, with one chromosome from each pair going into one of the two daughters cells

Question 21

What happens during the second meiotic division? (Meiosis 2)

Answer 21

• The chromosomes move apart
• At the end of meiosis 2, 4 daughter cells have usually been formed

Question 22

In humans, how many chromosomes does each daughter cell produced from meiosis contain?

Answer 22

23 chromosomes.

Question 23

What are 2 ways meiosis produces genetic variation among offspring?

Answer 23

• Independent segregation of homologous chromosomes
• New combinations of material and paternal alleles by crossing over

Question 24

What is genetic recombination by crossing over?

Answer 24

• In meiosis 1, when each chromosome lines up alongside its homologous partner…
• The chromatids of each pair become twisted around one another
• During this twisting process tensions are created and portions of the chromatids break off
• These broken portions might then re-join with the chromatids of its homologous partner
• Usually is the equivalent portions of homologous chromosomes that are exchanged
• In this way new genetic combination of maternal and paternal alleles are produced

Question 25

For meiosis, what equation determines the number of possible combinations for each daughter cell?

Answer 25

2^n, where n = the number of pairs of homologous chromosomes

Question 26

After meiosis is complete, what further increases genetic variety?

Answer 26

The random pairing of male and female gametes.

Question 27

What is binary fission?

Answer 27

Cell division in prokaryotic cells.

Question 28

Describe the process of binary fission:

Answer 28

• The circular DNA molecule replicates and both copies attach to the cell membrane
• The plasmids also replicate
• The cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm into two
• A new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids

Question 29

Can viruses undergo cell division?

Answer 29

No, as they are non-living.

Question 30

So how do viruses replicate?

Answer 30

• They attach to a host cell using the attachment proteins on their surface
• They then inject their nucleic acid into the host cell
• The genetic information on the injected viral nucleic acid then provides the ‚instructions’ for the host cell’s metabolic processes to start producing the viral components, nucleic acid, enzymes and structural proteins, which are then assembled into new viruses

Question 31

What are homologous chromosomes?

Answer 31

A pair of chromosomes, one maternal and one paternal, that have the same gene loci.