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 the cell cycle. The time taken for this cycle to occur varies greatly, from as short 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 Phase The 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 Phase The 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 Phase Gap 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 purpose We 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 purpose In 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 development Multicellular 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 development Development 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 repair The 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 repair These 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 and briefly summarise where mitosis occurs in the following organisms. - In mammals - In fungi - Some plants (Bryophytes) - Liverworts - Insects and other invertebrates

Programmed cell death Cells 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.