TOPIC 3B

Answer 1

The cell cycle is the regulated sequence of events that occurs between one cell division and the next

Answer 2

The cell cycle has three phases:

Interphase

Nuclear division (mitosis)

Cell division (cytokinesis)

Answer 3

The movement from one phase to another is triggered by chemical signals called cyclins

Answer 4

During Interphase the cell increases in mass and size and carries out its normal cellular functions, e.g. synthesising proteins and replicating its DNA ready for mitosis

Answer 5

Interphase consists of three phases:

G1 phase

S phase

G2 phase

Question 6

Nuclear division (mitosis)
Follows interphase

Referred to as the M phase – M stands for mitosis

Cell growth stops during the M phase

During mitosis, the two identical sister chromatids of each chromosome separates from each other and move to opposite poles of the cell

This ensures that each new nucleus that forms will contain the exact same genetic information as the original nucleus

Answer 7

Cytokinesis
Follows M phase

Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells

In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

Question 8

Examiner Tips and Tricks
Make sure you know the order of the phases of the cell cycle but also what specifically occurs during the different phases. Don’t forget, interphase is itself made up of three distinct stages (G1, S and G2) and you need to know what happens during each of these.

For example, an exam question might ask you to identify the stage of the cell cycle during which a cell would be producing the most mRNA molecules and explain why. The correct answer would be the G1 phase, as this is when protein synthesis is occurring and the production of mRNA occurs during transcription (the first part of protein synthesis).

Question 9

Mitosis
Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)

Although mitosis is, in reality, one continuous process, it can be divided into four main stages

These stages are:

Prophase

Metaphase

Anaphase

Telophase

Answer 10

Prophase
Chromosomes condense and are now visible when stained

The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere

The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)

Spindle fibres (protein microtubules) begin to emerge from the centrosomes (which consist of two centrioles in animal cells)

The nuclear envelope (nuclear membrane) breaks down into small vesicles

Question 11

Metaphase
Centrosomes reach opposite poles

Spindle fibres (protein microtubules) continue to extend from centrosomes

Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles

Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

Each sister chromatid is attached to a spindle fibre originating from opposite poles

Question 12

Anaphase
The sister chromatids separate at the centromere (the centromere divides in two)

Spindle fibres (protein microtubules) begin to shorten

The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Question 13

Telophase
Chromosomes arrive at opposite poles and begin to decondense

Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

The spindle fibres break down

Question 14

The significance of mitosis
The process of mitosis is of great biological significance and is fundamental to many biological processes, including:

The growth of multicellular organisms

The replacement of cells and repair of tissues

Asexual reproduction

Question 15

Growth of multicellular organisms
The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell

This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms

Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Question 16

Replacement of cells & repair of tissues
Damaged tissues can be repaired by mitosis followed by cell division

As cells are constantly dying they need to be continually replaced by genetically identical cells

In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut

Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts

Question 17

Meiosis I

The nucleus of the original ‘parent’ cell is diploid (2n) i.e. it contains two sets of chromosomes

Before meiosis I, these chromosomes replicate

During meiosis I, the homologous pairs of chromosomes are split up, to produce two haploid (n) nuclei

At this point, each chromosome still consists of two chromatids

Note that the chromosome number halves (from 2n to n) in the first division of meiosis (meiosis I), not the second division (meiosis II)

Question 18

Meiosis II
During meiosis II, the chromatids that make up each chromosome separate to produce four haploid (n) nuclei

At this point, each chromosome now consists of a single chromatid

Meiosis I and II
During meiosis, one diploid nucleus divides by meiosis to produce four haploid nuclei

Having genetically different offspring can be advantageous for natural selection

Meiosis has several mechanisms that increase the genetic diversity of gametes produced. The two main mechanisms are:

Independent assortment

Crossing over