2.6 Cell division, cell diversity and cellular organisation

Question 1

State what the cell cycle is and outline it’s stages.

Answer 1

•The cell cycle is the regulated cycle of division with intermediate growth periods
•The three phases are interphase, mitosis/meiosis and cytokinesis

Question 2

Outline what happens during interphase.

Answer 2

•Gap phase 1 - cell synthesises proteins for replication and the cell’s size doubles
•Synthesis phase - DNA replicates, chromosomes consist of 2 sister chromatids joined at a centromere
•Gap phase 2 - Organelles divide.

Question 3

What is the purpose of mitosis ?

Answer 3

•Mitosis produces 2 genetically identical daughter cells for growth, tissue repair and asexual reproduction.

Question 4

Name the stages of mitosis

Answer 4

•Prophase
•Metaphase
•Anaphase
•Telophase

Question 5

Outline what happens during prophase

Answer 5

•Chromosomes condense, becoming visible
•Centrioles move to opposite poles of the cell and spindle fibres form
•The nuclear membrane dissolves, leaving the chromosomes free in the nucleus

Question 6

Outline what happens during metaphase

Answer 6

•Sister chromatids line up at the cell equator attached to spindle fibres by their centromeres.

Question 7

What happens during anaphase ?

Answer 7

•Spindle fibres contract, centromeres divide
•Sister chromatids separate into two distinct chromosomes and are pulled to opposing poles of the cells
•Spindle fibres break down

Question 8

Outline what happens during telophase.

Answer 8

•Chromosomes decondense, becoming invisible again
•New nuclear envelopes form around each set of chromosomes
•This leaves two nuclei each with 23 pairs of chromosomes

Question 9

What happens during cytokinesis ?

Answer 9

•Cell membrane cleavage furrow forms
•Contractile division of cytoplasm

Question 10

How is the cell cycle regulated ?

Answer 10

•Cell signalling proteins ensure damaged cells cannot progress to next stage of division

Question 11

What is meiosis ?

Answer 11

•A form of cell division that produces four genetically unidentical haploid cells (gametes)

Question 12

What happens during meiosis I ?

Answer 12

•2 Homologous pairs come together to form bivalents
•DNA crosses over (is exchanged between homologous pairs). This occurs at chiasmata
•Cell divides into two. Homologous chromosomes separate randomly. Each cell will have either the maternal or paternal component of the homologous pair.

Question 13

What are homologous chromosomes ?

Answer 13

•A pair of chromosomes with genes at the same loci.
•1 maternal and 1 paternal
•Some alleles will be different between each chromatid.

Question 14

What happens during meiosis II ?

Answer 14

*The sister (identical) chromatids separate and go into different cells when the cell divides.
*This leaves the gamete with 23 chromatids.

Question 15

How does meiosis produce genetic variation ?

Answer 15

•Crossing over during meiosis I
•The independent assortment (random separation) of homologous chromosomes and sister chromatids
•Both of these result in new combinations of alleles.

Question 16

How do cells become specialised?

Answer 16

•Some genes in the cells DNA are expressed while others are silenced due to cell differentiation - caused by transcription factors
•Cells produce proteins that determine their structure and function

Question 17

What is a transcription factor ?

Answer 17

•A protein that controls the transcription of genes so that only certain parts of the DNA are expressed
•Transcription factors are used to specialise cells.

Question 18

How do transcription factors work ?