Topic D

Question 1

Ploidy and chromosome number

Answer 1

Every species has a characteristic number of chromosomes and chromosome sets

Humans have two sets of chromosomes
- 2n = diploid

Chromosome number
- Each set has 23 chromosomes for a total of 46

Question 2

Homologous Chromosomes

Answer 2

In a diploid (2n) cell, every chromosome has a homologue

Question 3

Features used to differentiate chromosomes

Answer 3

• Size
• Centromere position
• Other features

In this human karyotype note how size, banding patter differ, and how each chromosome has a homolog because humans are diploid

Question 4

Bacteria Cell Division

Answer 4

Bacteria reproduce asexually by binary fission
- Chromosome is replicated
- The cell divides such that each daughter cell receives a complete chromosome

Question 5

Why do cells need to divide in multicellular eukaryotes?

Question 6

Cell division requires and includes

Answer 6

Cell division requires that the genetic material is:
- Replication
— Prior to cell division
- Transmitted

Cell division includes
- Binary fission (bacteria)
- Mitosis (eukaryotes – both haploid and diploid cells)
- Meoisis (eukaryotes – only diploid cells)

Question 7

Cell division

Answer 7

DNA must be fully replicated before mitosis and meiosis
- Happens during S phase
- Doubles DNA content (C)
- Does not change ploidy
- Does not change chromosome number

Prior to cell division (mitosis or meiosis), DNA must be fully replicated during S phase of the cell cycle
- Chromosome vs. Chromatid
- Chromosome number does not
change through S phase

Question 8

Mitosis

Answer 8

Is an equational division
- Ploidy does not change
- Two daughter cells are produced
- Both haploid and diploid cells can undergo mitosis

Stages of Mitosis
- Prophase
- Metaphase
- Anaphase
— Reduction in DNA content occurs in anaphase
- Telophase and cytokinesis

Question 9

Meiosis

Answer 9

is a reduction division
- Ploidy is reduced from 2n to n
- Four meiotic products are produced
- Only diploid cells can undergo meiosis

Meiosis produces haploid cells with a single copy of each chromosome

Specialized cells called meiocytes can undergo meiosis
- Oocytes to produce eggs
- Spermatocytes to produce sperm

2 successive nuclear divisions
- Meiosis I = reduction division,
ploidy will be reduced from 2n to n
q Meiosis II = equational division, ploidy starts at n and stays at n

Question 10

Haploid cells

Answer 10

Haploid cells CAN NOT carry out meiosis
- Meiosis is a reduction division where ploidy is reduced

Haploid organisms CAN have meiosis in their life-cycle
- How?

Question 11

Cell division

Answer 11

Changes to DNA content in a diploid (2n) cell
- Prior to S Phase (G1) DNA content is 2 (C=2)
- Following S Phase (G2) DNA content doubles (C=4)
- Following Mitosis DNA content is reduced by half (C=2)
- Following Meiosis DNA content is reduced to 1 (C=1)

DNA content, chromosome number and ploidy
- G1: A cell is 2n=46; C=2; What will be after S phase?

After S phase: Cell is 2n=46; C=4
- DNA content (C) doubles after S phase

After Mitosis – two 2n=46; C=2 cells
- DNA content (C) is reduced from 4 to 2
- Ploidy unchanged - Mitosis is an equational division

Meiosis – four n=23; C=1 products
- DNA content (C) is reduced from 4 to 1
- Ploidy reduced to n - Meiosis is a reduction division

Question 12

DNA content

Answer 12

Often ploidy and chromosome number are easier to understand than DNA content, so let’s address DNA content in more detail

• DNA content is the mass of normalized to 1C for a haploid, unreplicated, genome
• A diploid, unreplicated, genome has a DNA content of 2C
• When the DNA of a diploid cell is replicated DNA content
  doubles from 2C to 4C
• When a 2n, 4C cell divides mitotically it produces two 2n, 2C products
• When a 2n, 4C cell divides meiotically is produces four n, 1C products

Question 13

Meiosis 1

Answer 13

Meiosis I
- reduction division

Prophase I
- Synapsis between homologous chromosomes to form bivalents
- Crossing over between non-sister chromatids
- Visible as chiasmata/chiasma

Metaphase I
- Bivalents line up

Anaphase I
- Reduction in ploidy from 2n to n
- Reduction in chromosome number (by half)
- Reduction in DNA content (C) from 4C to 2C

Question 14

Meiosis 2

Answer 14

Meiosis II (equational division)
- Prophase II
- Metaphase II – chromosomes line up at the metaphase plate q
- Anaphase II – DNA content (C) is reduced from 2C to 1C

Question 15

Meiosis and Mendel’s 1st Law

Answer 15

Mendel’s first law of equal segregation and meiosis
- Homologous chromosomes segregate equally from one another during meiosis
- Therefore, the two alleles of a gene, one present on each homolog, segregate equally from one another during meiosis

For example
- If a diploid cell with the genotype Aa undergoes meiosis
— 50% (0.5) A gametes
— 50% (0.5) a gametes

Question 16

Meiosis and Mendel’s 2nd Law

Answer 16

Mendel’s second law of independent assortment
and meiosis
- Different chromosomes assort independently of one another
- Therefore, the alleles of different genes assort independently of one another

For example, Gene A and B are on different chromosomes
- If a cell with the genotype Aa Bb undergoes meiosis
— What is the probability that a gamete is A? 50%
— What is the probability that a gamete is B? 50%
— What is the probability that a gamete is A and B? 25%

Question 17

Changes in chromosome number and structure

Answer 17

Mutation can happen on a small scale (point mutation) or a large scale

Errors during meiosis can result in changes to:
- Chromosome number (aneuploidy)
- Ploidy
- Chromosome structure (translocations, duplications, deletions, inversions)

Changes in ploidy
- Changes in the number of chromosome sets can also occur
- This may result in new species in a single generation

Chromosome structural changes
- Translocations
- Inversions
- Duplications and deletions

These changes may impact cell function

If they occur in the germline
- Can result in non-viable gametes, or early pregnancy loss
- Can alter the phenotype of the next generation in a variety of ways
- May be silent

If they occur in somatic cells
- May impact cell function
- May be silent

Question 18

Euploids and Aneuploidy

Answer 18

Euploids have the typical number of chromosomes for a species

Aneuploidy
- Aneuploidy is the result of nondisjunction during meiosis
- Aneuploid gametes usually not viable and can result in early pregnancy losses

Question 19

Summary

Answer 19

• Mitosis is an equational division which yields two genetically identical daughter cells.
• Meiosis is a reduction division which yields four cells, each with half the number of chromosomes as the parental cells.
• Haploid (n) cells can undergo mitosis to produce haploid products.
• Diploid cells (2n) can undergo mitosis to produce diploid products, or meiosis to produce haploid products.
• During meiosis each member of a homologous pair of chromosomes segregates to opposite poles –described as ‘equal segregation’.
• During meiosis the segregation of one homologous pair is independent of the segregation of other homologous pairs – described as “independent assortment”.
• Errors during meiosis can impact chromosome number and structure, which may have implications for phenotype.