Figure 1.15 The cell cycle

      The whole cell cycle takes approximately 24 hours, although this depends on which type of cell is involved. Mitosis usually only accounts for about an hour. Interphase is when no cellular division takes place. However, even during interphase, the cell needs to get ready for division so it increases in size. This stage is known as Gap 2 or G2. After division the cell needs to continue to grow so that it can achieve its optimum size; this is known as Gap 1 or G1.

      Normally cells can undergo a total of 80 mitotic divisions before the cell dies, although this is dependent on the age of the individual.

      Meiosis

      Each cell contains two sets of chromosomes which exist in pairs. Meiosis results in cell division that produces new cells with only half the chromosomal complement. This is needed for the formation of germ cells (sperm in men, ova in women) so that two germ cells can fuse to form a full chromosomal complement.

      Halving the full complement is achieved in two steps called meiosis I and meiosis II. Meiosis I is very similar to mitotic division in that two daughter cells are produced, both with 46 chromosomes. The main difference is that meiosis I takes much longer in comparison to mitosis and results in the ‘crossing over’ of chromosomal material (see Figure 1.16). Chromosomes ‘swap’ or exchange pieces of their structure with their partner chromosome before separating. This results in the daughter cells not having identical genetic material. This is the cause of genetic variability between individuals.

      Meiosis II does not involve chromosomal replication but does involve the stages of prophase, metaphase, anaphase and telophase where chromosomes separate, new nuclei are formed and the cell splits into two. At the end of meiosis II the cells contain 23 individual chromosomes.

Figure 1.16 Crossing over

      Differences between mitosis and meiosis

      The differences between mitosis and meiosis are illustrated in Figure 1.17 and Table 1.2.

Figure 1.17 Mitosis and meiosis

Table 1.2 The differences between mitosis and meiosis

Mitosis	Meiosis
one division	two divisions
results in 2 daughter cells	results in 4 daughter cells/gametes
genetically identical	genetically different
same chromosome number	chromosome number halved
occurs in all body cells	occurs only in germ cells
occurs throughout life	occurs only after sexual maturity
used for growth and repair	used for production of gametes

ACTIVITIES 1.2, 1.3 AND 1.4

      1.2. Explain why there is significant genetic variation as a result of meiosis but not of mitosis.

      1.3. Describe the phases of the cell cycle.

      1.4. Explain the reason why germ cells have to undergo meiotic division.

      GENETIC INFORMATION

      Chromosomes are made up of long chains of DNA (Deoxyribonucleic Acid) and protein molecules. It is the DNA within the chromosomes that holds all genetic information. The total length of the DNA within each cell is over 2m (6 feet) and, in order to fit within the cell’s nucleus, it has to exist in a tightly packaged form. This is achieved by the DNA being coiled around protein structures called histones (see Figure 1.18). The DNA wraps around eight histones to form a structure called a nucleosome. Thousands of nucleosomes are formed, which gives the DNA molecule the appearance of a string of beads. Further coiling of these nucleosome beads results in a shortened structure called a chromatin fibre. It is these tightly packaged chromatin fibres that make up chromosomes.

Figure 1.18 Histones, nucleosomes and chromatin fibre

      The DNA within the chromosomes contains coded instructions for the production of protein. The coded area for the production of a specific protein is called a gene.

      The structure of DNA

      The structure of DNA was discovered through X-ray diffraction back in 1953 by the Nobel Prize-winning scientists James Watson and Francis Crick. DNA is composed of bases, sugars and phosphates that combine together to form a double helix. The double helix shape looks like a twisted ladder. The ‘sides’ of the ladder are made of phosphates and sugars, while the ‘rungs’ of the ladder are made of bases. Only four different types of bases exist within the DNA:

      • Adenine (A);

      • Guanine (G);

      • Cytosine (C);

      • Thymine (T).

      DNA bases pair up with each other to form the ‘ladder rungs’ (see Figure 1.19). Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. Only these two types of base pairing exist in DNA. The order of the base ‘rungs’ along the DNA ladder varies but the base pairings are always complementary.

Figure 1.19 DNA bases

      The sequences of bases on one DNA strand can be deduced from the sequence on the opposite strand, because base pairing is always complementary. Each strand independently carries the information required to form a double helix. Therefore, to describe a DNA sequence, only the sequence of the bases in one strand is needed, for example ATTGCAAT, as the other strand is always complementary, i.e. TAACGTTA. Human DNA consists of about 3 billion bases, of which over 99 per cent of the sequence is identical in all people. These bases, within the DNA, form the code for the production of proteins.

      PROTEIN

      All the functions of the cell depend on protein. Protein maintains cell structure, acts as both intracellular and extracellular messengers, binds and transports molecules and acts as enzymes.

      Some proteins exist in every cell, such as the enzymes involved in glucose metabolism. Other proteins are highly specialised and are only found in specialised cells, such as the protein myosin, found only in muscle cells, or the protein insulin that is only produced in pancreatic islet cells.

      What are proteins?

      Proteins are made up of long chains of amino acids. There are only 20 different types of amino acids but, by varying the order and amount of amino acids in the chain, thousands of different proteins can be produced.

      Links within the chain of amino acids are called peptide bonds, while the chain itself is known as a polypeptide. A protein can contain one or more polypeptides. Both the structure and function of the protein depend on the sequence of the amino acids making up the polypeptide chains.

      In order to function, cells need

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