2.1.6 Cell Division, Cell Diversity and Cell Differentiation

Question 1

What does haploid mean?

Answer 1

• having only one set of chromosomes
• represented by the symbol ‘n’

Question 2

What is the significance of meiosis in life cycles?

Answer 2

• sexual reproduction increases genetic variation as it combines genetic material from two(usually) unrelated individuals of thr same species by fertilisatiion
• genetic variation within a population increases chances of survival when environment changes
• in many organisms, body cells are diploid, but for sexual reproduction to occur, they must produce haploid gametes

Question 3

What are homologous chromosomes?

Answer 3

• matching chromosomes, containing the same genes at the same loci
• may contain different alleles for some of the genes

Question 4

Briefly describe the main stages of meiosis

Answer 4

first meiotic division:

• prophase 1
• metaphase 1
• anaphase 1
• telophase 1

second meiotic division:

• prophase 2
• metaphse 2
• anaphase 2
• telophase 2

at the end, cytokinesis may occur

Question 5

What happens in prophase I stage of meiosis?

Answer 5

• the chromatin condenses and each chromosome supercoils
• they can take up stains and be seen with a light microscope
• nuclear envelope breaks down and spindle threads of tubulin protein form from the centriole in animal cells
• the chromosomes come toghether in their homologous pairs
• each member of the pair consists of two chromatids
• crossing over occurs where non-sister chromatids wrap around each other and may swap section to shuffle alleles

Question 6

Describe metaphase 1

Answer 6

• the pairs of homologous chromosomes, still in their crossed over state, attach along the equator of the spindle
• each attaches to a spindle thread by its centromere
• the homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell
• this is called independent assortment
• the way they line up in metaphses determines how they will segregate independently when pulled part during anaphase

Question 7

Describe anaphase 1

Answer 7

• the members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin threads of the spindle
• the centromeres do not divide, and each chromosome consists of two chromatids
• the crossed-over areas separate from each other, resulting in swapped areas of chromosome and allele shuffling

Question 8

Describe telophase 1

Answer 8

• in most animal cells, two new nuclear envelopes form around each set of chromosomes, and cell divides by cytokinesis
• there is then a short interphase when the chroomsomes uncoil
• each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids
• in most plant cells, the cell goes straight from anaphase 1 iinto prophase 2

Question 9

Describe prophase 2

Answer 9

• if the nuclear envelopes have reformed, they now break down
• chromosomes coil and condense, each one consisting of two chromatids
• chromatids of each chromosmoe are no longer identical, due to crossing over in prophase 1
• spindles form

Question 10

Describe metaphase 2

Answer 10

• the chromosomes attach, by their centromere, to the equator of the spindle
• the chromatids of each chromosome are randomly arranged
• the way they are arranged determines how chromatids separate during anaphase

Question 11

Describe anaphase 2

Answer 11

• the centromeres divide
• chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubulin threads of the spindle, towards oppposite poles
• chromatids are therefore randomly segregated

Question 12

Describe telophase 2

Answer 12

• nuclear envelopes form around each of the four haploid nuclei
• in animals, the two cells now divide to give their four haploid cells
• in plants, a tetrad of four haploid cells is formed