4-Mitosis and Meiosis

Question 1

Interphase

Answer 1

• DNA replication occurs
  -Organelles are synthesised
• Increased rate of respiration
• Increased protein synthesis
  Evidence:
  -Chromosomes aren’t visible
• Nuclear membrane and nucleolus still present

Question 2

Prophase

Answer 2

• Chromosomes condense/spiralise - become shorter and thicker
  -Nuclear envelope breaks down
  Evidence:
• Sister chromatids together
  -Highly disorganised

Question 3

Metaphase

Answer 3

• Spindle fibres form which centromeres attach to
• Chromosomes move along spindle fibres to middle of cell
  Evidence:
• Chromosomes are in the middle of the cell

Question 4

Anaphase

Answer 4

• Centromere splits
• Sister chromatids separate and are pulled apart to opposite ends of the cell when spindle fibres contract
  Evidence:
• Set of chromatids at both ends of the cell

Question 5

Telophase

Answer 5

• Nuclei reform
• DNA becomes chromatin
  (loose)
• Cytoplasm splits (Cytokinesis)
• Two Genetically identical cells formed
  Evidence:
• Chromosomes not visible
• Two cells appearing
• Two nuclei appearing

Question 6

Importance of meiosis

Answer 6

• During fertilisation the diploid number is restored which ensures that the diploid number remains the same after every generation
• Produces genetic variation in offspring - increases chance of survival if environment changes

Question 7

Meiosis 1

Answer 7

• Homologous chromosomes pair up - bivalents form
• Crossing over takes place at chiasmata -Homologous chromosomes twist around each other - section of DNA with blocks of alleles break off, swap positions and reattach- creates new combination of alleles
• Daughter cells contain one chromosome from each homologous pair randomly because maternal and paternal chromosome line up randomly within each pair- diff combo of chromosomes produced in gametes- diff combo of alleles - independent assortment

Question 8

Meiosis

Answer 8

• Metaphase II, Anaphase II, Telophase II
• Chromatids on each chromosome are separated producing 4 haploid cells

Question 9

How can mutations in no. of chromosomes happen

Answer 9

-Failure of homologous pair of chromos to separate during 1st division of meiosis
- Failure of pair of sister chromatids to separate during 2nd division of meiosis
- Chromosome non- disjunction