mitosis and meiosis

Question 1

What is mitosis?

Answer 1

mitosis – division that maintains the same number of chromosomes, producing 2 new cells that are genetically identical to the parent cell.

Question 2

What is meiosis?

Answer 2

meiosis – division that halves the number of chromosomes, producing 4 haploid cells, each genetically different to the parent cell.

Question 3

What is ploidy

Answer 3

Ploidy – how many complete sets of chromosomes a cell has.

Question 4

What is haploid

Answer 4

haploid – the number of chromosomes found in gamete cells (sperm/ova which is half the normal ploidy of the cells that produce the gametes by meiosis. e.g. Homo sapiens L. is diploid (2 sets of chromosomes) and our haploid sperm cells have 1 set).

Question 5

What is monoploid

Answer 5

one set of chromosomes.

Question 6

What is diploid

Answer 6

Two sets of chromosomes

Question 7

What is an example of a when species that is not all diploid?

Answer 7

Eusocial Insecta e.g. males (drones) are haploid, developing
from unfertilised eggs; females (queens and workers) are diploid, developing from fertilised eggs. Queens are selected for by feeding larvae with royal jelly, which allows them to develop active ovaries.

Question 8

What is a nucleolus?

Answer 8

A region within the nucleus (not membrane bound) in which ribosomes are made and which has some regulatory
roles. Often dozens of nucleoli in every nucleus!

Question 9

overview of interphase

Answer 9

This is the stage in which a cell spends most of its life.
Cell is actively consuming nutrients and making ATP and [H]* (“reducing equivalents”)old-school term for NADH,NADPH, FMNH2 FADH2 reduced cytochromes, ubiquinol etc (i.e. electron carriers in their reduced state, as
written here)

• three checkpoints occur that check each stage of interphase

Question 10

What are the three checkpoints of interphase?

Answer 10

Gap 1 (G1) in which the cell grows and metabolises normally. It grows to double the normal size and makes more organelles and more cytoplasm. Huge amounts of protein synthesis. If the cell is NOT going to divide by mitosis, it leaves G1
and enters the Gap 0 (G0) stage and just stays there. All
neural tissue is in G0

2. Synthesis (S) in which a lot more DNA is synthesised – enough to double the chromosome number e.g. in a normal diploid human cell (46 chromosomes), there are 92 chromosomes when S ends. Note the chromosomes don’t yet condense into their classical shapes – they are still just
   chromatin.
3. Gap 2 (G2) in which the cell keeps growing. Mitochondria (and plastids) now divide by binary fission to double their numbers.
   Cell in interphase (end of G2).

Question 11

What happens in prophase?

Answer 11

Mitosis formally commences.
chromosomes condense out of chromatin into their classical shapes.
All the new DNA made in S of interphase condenses into a duplicate set of chromosomes. Condensins (large protein complexes) assist this.
-This is a pair of identical sister chromatids joined by a centromere.
Each chromatid is a copy of the other so each chromosome in prophase actually contains twice as much DNA as in interphase.
* nucleoli break down. No more ribosomes (thus proteins) can’t be made. Cell is now focused on division and day-to-day metabolism stops.
* centrosomes (replicated in interphase) move apart. Each
centrosome (microtubule organisation centre) is made up of two centrioles at 90°. They move to opposite poles of the cell and microtubule activity at poles increases.
* microtubules reorganise.
In interphase these act like scaffolding. They now break down into radial arrays of (short) aster microtubules which make up asters centred on each centrosome. Longer sets of
spindle microtubules extend across the cell and form the basis of the mitotic spindle

Question 12

What is the prometaphase?

Answer 12

before metaphase
* nuclear envelope breaks down into vesicles.
* kinetochores form – disc-shaped pads that bind to centromere on each chromosome. Have protein extensions that allow binding to microtubules
* kinetochore microtubules extend from centrosomes and bind to kinetochores.
Chromosomes can be seen visibly ‘juddering’ in the cell as this happens.
* polar microtubules are formed, extending from centrosomes – the mitotic spindle is now complete.

Question 13

What occurs in metaphase?

Answer 13

• chromosomes align along the equator of
  the cell forming the so-called ‘metaphase
  plate’. Chromosomes are pulled back and forth until they form a perfect line.
• cell cannot enter anaphase until every
  chromosome pair (each chromatid is a
  copy, remember?) is attached to a
  kinetochore microtubule and aligned along
  the equator of the cell.

Question 14

What occurs in anaphase?
What protein prevents anaphase?

Answer 14

• the protein securin blocks anaphase from
  starting until everything is ready, and the
  anaphase-promoting complex destroys it,
  allowing this to proceed.
• anaphase A: kinetochore microtubules pull
  on the chromatids pulling them apart and
  moving them towards the poles. Chi-shape is
  lost.
• anaphase B: microtubules move towards the
  middle of the cell and push against one
  another, pushing chromatids nearer to the
  poles.

Question 15

Explain telophase

Answer 15

Nuclear envelopes reassemble around each set of chromatids at the poles of the cell.
* once nuclear envelopes re-form, the chromatids condense into chromatin and the nucleoli reform.
* mitotic spindle is disassembled.
* cytokinesis occurs – a myosin and actin ring around the equator of the cell contracts and splits the cell into two daughter cells.

Question 16

explain meiosis I – prophase I

Answer 16

• crossing-over events occur but otherwise the same
  as in mitosis.
• leptonene phase sister chromatids become tightly associated they come together and join by centromeres and stick together the telomeres also stick and some elements of the synaptonemal complex assemble between each pair of chromatids (proteins that connect the chromatids along their length).
• zygotene phase. Homologous chromosomes now line up
  next to one another.
• pachytene phase- homologous recombination occurs by
  crossing-over. Chiasmata form at contact points. Non-sister
  chromatids exchange information – results in
  chromatids that are still complete and same size but
  have swapped information with another of same
  size.
• diplotene phase the synaptonemal complex degrades. Chiasmata DO NOT degrade!

Question 17

Key differences in Meiosis I – metaphase I, anaphase I, telophase I to mitosis.

Answer 17

• happens much like mitosis but with chromosomes at the poles of the cell.
  *It is not single chromatids at each pole now but pairs of them chromosomes.
• sometimes the two daughter cells produced will have a little rest before proceeding after telophase I – this is
  called interphase II or interkinesis.

Question 18

What happens during G2 of interphase in a member of the Rhodophyta?

Answer 18

cell grows and mitochondria and rhodoplasts divide by binary fission.

Question 19

What are the centromere and chaisma?

Answer 19

the centromere is a protein structure that links the two
sister chromatids and acts as the binding-site for the kinetochore.
A chiasma is the point at which two non-sister chromatids from homologous chromosomes cross-over during meiosis to exchange DNA.

Question 20

What is the centrosome?
Describe the types of microfilament extending therefrom in mitosis?

Answer 20

the centrosome is the microtubule organising centre of the cell and is composed of two centrioles (protein complexes). In mitosis, the centrosomes produce aster microtubules that make up the aster and then spindle microtubules that make up the mitotic spindle along with kinetochore microtubules that bind kinetochores on chromosomes.