Cell and Nuclear Division

Question 1

Similarities between a pair of homologous chromosomes

Answer 1

• same length
• same genes at the same loci
• same position of centromeres

Question 2

Differences between a pair of homologous chromosomes

Answer 2

• for the same gene loci, they may have slightly different nucleotide sequences resulting in different alleles
• since one homolog is derived from maternal parent and the other is derived from paternal parent, one allele is of maternal origin and one is of paternal origin

Question 3

Describe what happens during crossing over between non-sister chromatids

Answer 3

• crossing over between non-sister chromatids of homologous chromosomes
• chiasma formation during prophase I of meiosis
• breaking of a corresponding DNA segment
• exchange of corresponding DNA segment between non-sister chromatids
• rejoining of DNA segment to the other chromatid
• separating alleles of linked genes and creating new allelic combinations

Question 4

What happens during prophase

Answer 4

• chromatin fibres become more TIGHTLY COILED, condensing into CHROMOSOMES
• each duplicated chromosome appears as TWO SISTER CHROMATIDS joined at their centromeres
• CENTROSOME organise microtubules into SPINDLE FIBRES
• ASTERS are the radial array of short microtubules extending from each centrosome
• centrosomes MIGRATE to OPPOSITE POLES of the cell by lengthening of microtubules
• NUCLEOLUS DISPERSE and seems to have disappeared
• NUCLEAR ENVELOPE FRAGMENTS

Question 5

What happens during metaphase (longest phase in mitosis)

Answer 5

• microtubules from centrosome are attached to KINETOCHORE at centromere of each chromosome, becoming KINETOCHORE MICROTUBULES
• CENTROMERES of chromosomes are ALIGNED along the METAPHASE PLATE by microtubule
• non-kinetochore microtubules interact with those from the opposite pole of the spindle

Question 6

What happens during anaphase (shortest phase in mitosis)

Answer 6

• CENTROMERE SEPARATES and two sister chromatids separate, becoming TWO DAUGHTER CHROMOSOMES
• daughter chromosomes MIGRATE TOWARDS OPPOSITE POLES OF THE CELL, with the CENTROMERE leading the way as KINETOCHORE MICROTUBULES SHORTEN
• NON-KINETOCHORE MICROTUBULED lengthen, leading to cell elongation

Question 7

What happens during telophase

Answer 7

• NUCLEAR ENVELOPE REFORMS from the fragments of the endomembrane system to form TWO NUCLEI
• NUCLEOLUS reappears
• chromosomes become LESS CONDENSED to form chromatin
• microtubules disperse by DEPOLYMERISING

Question 8

What happens to the nuclear envelope during mitosis

Answer 8

• during prophase, the nuclear envelope FRAGMENTS and appears to have disappeared
• this is to allow the MICROTUBULES from the centrosome to have access and ATTACH to the KINETOCHORE at centromere of chromosomes and SEPARATION OFSISTER CHROMATIDS towards opposite poles of the cell during anaphase
• during telophase, the nuclear envelope REFORMS from the fragments of the endomembrane system round the complete set of chromosomes at TWO POLES OF THE CELL to form TWO NUCLEI
• this facilitated the DIVISION OF CYTOPLASM which starts after telophase to form two daughter cells with genetically identical nuclei

Question 9

Why does the chromosome appear as a double arm structure

Answer 9

• each DNA molecule undergoes SEMI-CONSERVATIVE REPLICATION during SYNTHESIS PHASE OF INTERPHASE
• gives rise to two DNA molecules which CONDENSE during prophase to form TWO GENETICALLY IDENTICAL SISTER CHROMATIDS held together at the centromere

Question 10

Outline the roles of centromeres

Answer 10

• constricted region of satellite DNA that holds two sister chromatids together
• kinetochore proteins bind onto centromeres to allow attachment by spindle fibres
• spindle fibres can attach to the centromeres via kinetochore to align centromere of chromosomes along metaphase plate during metaphase

Question 11

Difference in behaviour of chromosomes in meiosis and chromosomes in mitosis

Answer 11

• homologous chromosomes form bivalents in meiosis during prophase I but homologous chromosomes do not pair up in mitosis during prophase
• crossing over occurs during prophase I of meiosis but it does not occur in prophase of mitosis
• homologous pairs of chromosomes line up at the equator during metaphase I in meiosis chromosomes line up singly during metaphase of mitosis
• homologous pairs of chromosome separate in anaphase I of meiosis but sister chromatids separate in anaphase of mitosis

Question 12

How are homologous chromosomes arranged during metaphase I of meiosis

Answer 12

• the bivalents align themselves at the metaphase plate, with one chromosome in each pair facing each pole
• the arrangement of chromosome of each bivalent is independent of the arrangement of the other bivalents
• both chromatids of one homolog are attached to kinetochore microtubules from one pole and those of the other homolog are attached to microtubules from the opposite poles
• non-kinetochore microtubules interact with those from the opposite pole of the spindle

Question 13

Distinguish between haploid and diploid

Answer 13

• haploid cells contain one set of chromosomes while diploid cells contain two sets of chromosomes, one maternal and one paternal
• gametes are haploid cells while somatic cells are diploid
• haploid cells have half the number of chromosomes to diploid cells

Question 14

Distinguish between centromere and centriole

Answer 14

• centromere consists of satellite DNA whereas centriole composed of nine sets of triplet microtubules arranged in a ring
• centromere holds sister chromatids together whereas centrioles organise microtubules into spindle fibres

Question 15

Outline what happens to a chromosome between the end of anaphase and the start of the next mitosis

Answer 15

• chromosomes begin to decondense into chromatin during telophase
• during interphase, transcription takes place, produce mRNA, synthesis of proteins
• during synthesis phase of interphase, semi-conservative replication takes place to produce new DNA molecule

Question 16

Why does the chromosome number change during formation of gametes

Answer 16

• chromosome number halved (from diploid to haploid)
• to restore diploid number at fertilisation/avoid chromosome number doubling in every generation

Question 17

Significance of mitosis

Answer 17

• confers genetic stability between generations of cells
• each parent cell produces two genetically identical daughter cells with the same number and types of chromosomes as parent nucleus
• daughter cells have identical genetic information as parent cell due to semi-conservative replication of parental DNA during synthesis phase of interphase
• this enables growth, repair and asexual reproduction
• cells that are lost, damaged or worn-out are replaced by genetically identical cells

Question 18

Checkpoints of mitotic cell cycle

Answer 18

G1 -> Synthesis -> G2 -> Mitosis
- G1 checkpoint ensures that cell size is adequate, and there are sufficient nutrients available and growth factors present for cell to undergo mitosis
- G2 checkpoint ensures that the cell size is adequate and that semi-conservative replication has been completed successfully
- Metaphase checkpoint ensures that all chromosomes are attached to spindle fibres at the metaphase plate before proceeding to anaphase

Question 19

Significance of meiosis

Answer 19

• production of haploid gametes which is important for fertilisation in sexual reproduction where fusion of gametes from different parents occurs to for zygote
• maintenance of chromosomal number in offspring and prevent doubling of chromosomal numbers during fertilisation
• generates genetic variation in offspring by producing recombinant gametes -> crossing over between non-sister chromatids of homologous chromosomes + independent assortment of homologous chromosomes

Question 20

When does non-disuunction occur

Answer 20

• a pair or pairs of homologous chromosomes fail to separate during anaphase I of meiosis
• sister chromatids fail to separate during anaphase II of meiosis