5.2 - Cell Replication

Question 1

Mitosis

Answer 1

• Forms 2 identical cells from 1 parent cell
• Used for growth and repair of dead/damaged cells
• Used for asexual reproduction in which offspring are genetic clones of their parent

Question 2

Meiosis

Answer 2

• ## Creates gametes that are genetically unique

Question 3

Cell cycle

Answer 3

• Series of events that occur over the life of a eukaryotic cell
• 2 main stages: interphase and mitosis/cytokinesis (cell division)

Cells spend most of their lives in interphase.

Phases:

• G0: “resting” - the cell is performing its function
• G1 cell growth: cellular contents excluding chromosomes are duplicated
• S synthesis: each chromosome is duplicated
• G2 proofreading: duplicated chromosomes are checked for errors and necessary repairs are made
• M mitosis: nucleus divides
• Cytokinesis: cytoplasm divides, forming 2 diploid daughter cells

Question 4

Interphase

Answer 4

• DNA is not yet compacted into chromosomes, rather it is spread out and called “chromatin”

Stages

• Resting: Gap 0 (G0)
• Interphase: Gap 1 (G1), Synthesis (S), Gap 2 (G2)

Question 5

Chromosome/chromatid/chromatin

Answer 5

2 chromatids: a chromosome
1 chromatid: made of chromatin fibres

Chromatin: DNA wrapped around histone proteins
When chromatin is condensed, it becomes chromatid.

In increasing order of complexity: chromatin → chromatid → chromosome

Question 6

Interphase

Gap 0

Answer 6

• Occurs in the Resting phase
• Cell has left the cycle and stopped dividing
• Usually occurs within or outside of G1 phase
• Allows the cell to carry out its other specialised functions

Question 7

Interphase

Gap 1

Answer 7

• First stage of interphase
• Cells increase in size
• Organelles duplicate (cell contents)
• Has a checkpoint control mechanism that ensures everything is ready for DNA synthesis

Question 8

Interphase

Synthesis

Answer 8

• 2nd stage of Interphase
• DNA replication occurs (in preparation for mitosis)

Question 9

Interphase

Gap 2

Answer 9

• Final stage of Interphase
• Cell continues to grow with replicated DNA
• Another checkpoint ensures everything is ready for mitosis
• Checkpoints are controlled by tumour suppressors, transcription genes, proteins and enzymes
• If something is wrong in G2, apoptosis occurs: programmed cell death

After Synthesis phase in Interphase, the cell DOES NOT divide immediately. The time between DNA replication and actual cell division is separated by Gap 2

Question 10

Prophase

Answer 10

• 1st stage of mitosis
• Chromatin material shortens and thickens by coiling, and DNA separates out into chromosomes
• Each chromosome contains 2 copies of DNA
• Each copy is called a sister chromatid, joined by a single centromere
• Spindle fibres begin to form
• Nuclear membrane distintegrates

At this stage, DNA REPLICATION has already occured (in the synthesis phase)

Question 11

Metaphase

Answer 11

• 2nd stage of mitosis
• Chromosomes align along the equator of the cell (metaphase plate)
• Each sister chromatid attaches to a spindle fibre originating from oppsosite poles

Question 12

Anaphase

Answer 12

• 3rd stage of mitosis
• Centromere are cleaved, allowing the sister chromatids to separate
• Spindle fibres pull chromsomes by their centromeres to opposite sides of the cell

Question 13

Telophase

Answer 13

• 4th and final stage of mitosis
• Daughter chromosomes gather at opposite poles of the cell
• Spindle fibre breaks down
• Nuclear membrane and nucleolus reappear
• Chromosomes also uncoil into chromatin again.

Question 14

Cytokinesis

Answer 14

• Division of the cytoplasm occurs
• Begins in the late stages of telophase in mitosis
• Cytoplasm furrows and splits into 2 daughter cells, as the cell membrane starts constricting
• A contractile ring made of microfilamnets causes the cell to constrict in the centre, just beneath the plasma membrane
• Contractile ring shrinks at the equator of the cell, pinching the plasma membrane inward → forms a cleavage furrow

Question 15

Centriole

Answer 15

Produce spindle fibres

Question 16

Telomere

Answer 16

• A region at the end of a chromosome that shortens as we age
• Once the telomeres reach a certain predeterimined length, the cell stops dividing and this leads to cell death or senescence
• There is a variation in the length of telomeres that people are born with and the rate at which they shorten