Cellular Replication and Variation

Question 1

Karyograms

Answer 1

Karyograms depict chromosomes arranged in their homologous pairs. There are 44 (22 pairs of) autosomes and 2 (1 pair of) sex chromosomes.

Question 2

Homologous Chromosomes

Answer 2

Matching pairs of chromosomes. One member of each pair originates from a female parent (maternal copy) and one from a male parent (paternal copy). They are the same length, banding pattern, have a centromere at the same position and contain the same genes in the same loci (positions).

As diploid cells, contain homologous pairs of chromosomes, mean that diploid cells also contain two copies of each gene (one on the maternal and one on the paternal copy of the chromosome)

Possible roles of Homologous chromosomes:

They separate during Meiosis 1

This allows for the recombination and random assortment of genetic material from a mother and father into new cells (during meiosis) ie. they allow for the mixing of genetic information from one generation to the next.

Question 3

Diploid cells

Answer 3

(2n) A cellular condition where there are two of each type of chromosome present in the nucleus of the cell (eg. zygote and somatic cells).

Question 4

Haploid cells

Answer 4

(n) A cellular condition where there is only one of each type of chromosome present in the nucleus of the cell (eg. sex cells)

Question 5

Sex Chromosomes

Answer 5

Chromosomes that contain genes that encode sex traits such as the X and Y chromosomes

Question 6

Autosomes

Answer 6

Non-sex chromosomes. In a somatic cell (body cell) of humans. Autosomes exist as homologous pairs but sex chromosomes may not.

Question 7

Duplicated Chromosomes

Answer 7

Contain two chromatids (containing identical DNA sequences) that are produced via DNA replication and are held together for a time by the centromere.

Question 8

The major stages in the cell cycle

Answer 8

Interphase
Nuclear Division
Cytokinesis

Question 9

The major stages in the cell cycle (Interphase)

Answer 9

Resting stage has 3 major stages:

G1 Phase- cell growth and enlargement, cell organelles and centrioles replicate.

S Phase- synthesis /replication of new DNA. Chromosomes duplicate.

G2 Phase- DNA begins to condense and enzymes required for mitosis are produced.

Question 10

The major stages in the cell cycle (Nuclear Division)

Answer 10

Nuclear Division (M) - Mitosis or Meiosis… contain 4 major stages:

Prophase
Metaphase
Anaphase
Telophase

Mitosis occurs during cell growth and repair in somatic cells whereas Meiosis occurs during sex cell formation in the gonads.

Question 11

The major stages in the cell cycle (Cytokinesis)

Answer 11

Entire cell divides & organelles / cytoplasm seperates (often occurs at the same time as Telophase at the end of M and start of G1 phases). In animal cells a cell furrow forms while in plant cells a cell plate forms.

Question 12

Mitosis

Answer 12

Occurs during growth and repair in somatic (body) cells.

There are 4 major phases that occur in a set sequence (PMAT).
(Prophase, Metaphase, Anaphase, Telophase)

Daughter cells are identical and are diploid (have 2 sets of chromosomes ie. same number and types of chromosomes as parent cell)

Question 13

Phases of Mitosis

Answer 13

1. Prophase: nuclear membrane disintegrates, spindle fibres begin to form as centrioles move to poles (parts of centromeres (kinetechores) attach to separate spindle fibres)
2. Metaphase: duplicated chromosomes align along the equator of the cell
3. Anaphase: chromatids from duplicated chromosomes separate & move towards the poles
4. Telophase: new nuclear membrane forms around new nuclei

Question 14

Meiosis

Answer 14

Occurs during sex cell formation inside germ cells in gonads (testes and ovaries).

The purpose of Meiosis is to produce haploid sex cells or gametes. (Because at fertilisation, two haploid gametes fuse to produce a diploid zygote)

Meiosis has two divisions:

Meiosis I- the Reduction Division (where homologous chromosomes pairs separate, halving chromosome number)

Meiosis II- the Mitotic Division (where sister chromatids in cells separate to produce 4 haploid daughter cells).

Both of these divisions involve the 4 major phases seen in mitosis (PMAT).

Daughter cells are not identical. Four haploid daughter cells are produced. Each have 1 set of chromosomes (ie. half the number of chromosomes as the parent cell).

Question 15

Phases of Meiosis

Answer 15

Prophase I: nuclear membrane disintegrates, duplicated homologous chromosomes pair up (synapsis occurs producing a tetrad) and attach to the same spindle fibre. Crossing over between some pairs occurs at this time.

Metaphase I: homologous chromosome pairs independently assort and align along the equator of the cell.

Anaphase I: homologous chromosome pairs separate & move towards opposite poles.

Telophase I: haploid nuclei form (new nuclear membranes form around two new nuclei).

(There is no interphase between meiotic divisions)

Prophase II: nuclear membranes disintegrate spindles form. Duplicated chromosomes attach to separate spindle fibers.

Metaphase II: Duplicated chromosomes align along the equator of the cell.

Anaphase II: Centromeres break and sister chromatids separate & move towards opposite poles.

Telophase II: haploid nuclei form (new nuclear membranes form around two new nuclei).

Question 16

Contributors to Variation in Offspring

Answer 16

Crossing over between homologous chromosomes leading to recombination of genetic material.

Independent assortment of chromosomes (and the genes they contain) during meiosis.

Random pairing of gametes at fertilisation

Question 17

Spermatogenesis and Oogenesis

Answer 17

Meiosis is the core nuclear division process that occurs during spermatogenesis (sperm formation) and oogenesis (egg formation). While there are some major similarities in these processes there are also some major differences.

Question 18

Spermatogenesis and Oogenesis Similarities

Answer 18

Meiosis is the core nuclear division process

Question 19

Spermatogenesis and Oogenesis Differences

Answer 19

Spermatogenesis occurs in testis, oogenesis in ovaries

Spermatogenesis has life long production of gametes, producing millions. Oogenesis has a fixed amount, only around 400

Spermatogenesis has four gametes per cell, oogenesis has one.

Spermatogenesis begins at puberty. Oogenesis begins with fetal development.

Spermatogenesis has continuous gamete production, gametes released at any time. Oogenesis has monthly gamete production, gametes released monthly. Spermatogenesis has uninterrupted meiotic divisions, Oogenesis has interrupted meiotic divisions.

Question 20

Sister chromatids

Answer 20

A sister chromatid refers to the identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be ‘one-half’ of the duplicated chromosome.