Cell replication Chapter 4. How long is a cell cycle? The time taken for a newly formed cell to mature and then give rise to two new cells is called.

Cell replication

Chapter 4

How long is a cell cycle?The time taken for a newly formed cell to

mature and then give rise to two new cellsis called the cell cycle. The time taken forthis cycle to occur varies greatly, from asshort as 20 minutes to as long as several

weeks. But generally the time lasts

• 10 to 30 hours in plants and• 18 to 24 hours in animals.

S PhaseThe phase between successive mitosis is called interphase and it is during a restricted period of

interphase, termed the S (for synthesis) period, that DNA is replicated in preparation for reproduction of

the cell.

Synthesis (S) phase

G PhaseThe S period is flanked by G (gap) phases during which

cell growth takes place. The G phase also seems to be at which the cell checks its DNA for mistakes

(shown as ‘checkpoints’ in fig 4.9)

Gap (G) periods

G PhaseGap 1 consists of an examination for any mistakes

in DNA that may have occurred in replication.

In Gap 2, cells check for mistakes that may have occurred during the S phase.

If the cell doesn’t get the ok to continue, it exits the cycle.

How many chromosomes?Humans body cells contain 46 chromosomes. In most

species of mammals, the males and females have the same number of chromosomes.

It is important to note that the size of an animal and its chromosome number are not related!

Quick check questions 1-5 on page 84

Mitosis – the purposeWe now know that mitosis is a division of the nucleus

by which genetic information is accurately copied and passed onto both daughter cells.

But what is the purpose of cell division???

How many chromosomes?The collection of chromosomes includes a pair of sex

chromosomes (X and Y in males and X and X in females). The remaining chromosomes are called

autosomes, and comprise of 22 homologous pairs. When chromosomes of a cell are paired in this way,

the cell is said to be diploid.

Mitosis – the purposeIn multicellular organisms, the purposes of cell

replication are growth, development, maintenance and repair. In unicellular eukaryotes, cell replication involving mitosis and cytokinesis is a simple means of reproduction – the cell’s production of genetically

identical copies of itself.

Growth and developmentMulticellular organisms grow in size by further

increasing the number of their cells through repeated cell replications. As the new individual

continues to develop, new cells become specialised for different purposes, such as

muscle, blood and bone in animals, or photosynthesis and transport in plants. More

replications follow and the cells become organised into the tissues that form the body of

the organism.

Growth and developmentDevelopment involves a balance between cell

replication and cell death. Some cells are designed to undergo a ‘programmed cell death’, like the skin

tissues that form the webbing between the fingers of a developing feotus. This topic will be discussed later.

Maintenance and repairThe activities of maintenance and repair of tissues require that new cells are produced to replace those damaged either during their normal function, such as the regular death of the surface layer of the lining of

the gut, or through injury.

Maintenance and repairThese cells are produced by mitotic cell division. The extent to which cells can be generated differs greatly

between organisms. For example, starfish can produce an entirely new organism by cell replication of a single arm. On the

other hand, a nematode worm, once hatched, cannot produce any new cells at all.

Growth, specialisation and death

Mitotic cell division is programmed to occur rapidly in embryos. As growth continues, a stage is reached

where individual cells begin to become different from one another, specialised for particular functions. This process, which is called specialisation (also called

differentiation), is under control of the genes.

Growth, specialisation and death

The zygote contains all the genes required to produce every type of cell, but in specialised

cells, only some genes are active.

Cell specialisation is found in all multicellular organisms, as cells are more efficient if they

focus on a single function rather than many. And obviously, the function of specialised plant cells differs greatly from the function of specialised

animal cells.

Growth, specialisation and death

For example;• the gene hemoglobin is active in developing red

blood cells• in gland cells, the gene that codes for a particular

enzyme or hormone is active• in nerve cells, genes that control the production

of neurotransmitter molecules are active• in lymphocytes, genes that produce antibodies

are active.

Where does mitosis occur?Read pages 85-88 and briefly summarise where mitosis

occurs in the following organisms.

- In mammals

- In fungi

- Some plants (Bryophytes)

- Liverworts

- Insects and other invertebrates

Programmed cell deathCells have controlled internal mechanisms which give

the instruction was to when the cell is to ‘self destruct’.

This programmed cell death is called apoptosis.

Apoptosis and cell replication are regulated by the body of a multicellular organism so that cell death and cell

replication are in balance.

Cancer can be the result of uncontrolled cell replication.

Chapter questions• Quick-check questions 6-8 page 88• Biochallenge questions 1 and 2 only page 90• Chapter review questions

3 and 6.