In sexual reproduction, a new organism starts life as a single cell called a zygote. The instructions that dictated which cells were to become liver, or muscle or bone must all have been present in this first cell. To understand how these ‘instructions’ are passed from cell to cell, what is important is that when the zygote divides and produces an organism consisting of thousands of cells. This type of cell division is called mitosis. It does not take place only in a zygote but occurs in all growing tissues.
Mitosis
When a cell is not dividing, there is not much detailed structure to be seen in the nucleus even if it is treated with special dyes called stains. Just before cell division however, a number of long, thread-like structures appear in the nucleus and show up very clearly when the nucleus is stained. These thread-like structures are called chromosomes. Although they are present in the nucleus all the time, they show up clearly only at cell division because at this time they get shorter and thicker.
Each chromosome is seen to be made up of two parallel strands, called chromatids. When the nucleus divides into two, one chromatid from each chromosome goes into each daughter nucleus. The chromatids in each nucleus now become chromosomes and later they will make copies of themselves ready for the next cell division. This is called replication, because each chromosome makes a replica of itself.
Human cells contain 46 chromosomes, found as 23 pairs. Mitosis will be taking place in any part of a plant or animal which is producing new cells for growth or replacement. Bone marrow produces new blood cells by mitosis. The epidermal cells of the skin are replaced by mitotic divisions in the basal layer; new epithelial cells lining the alimentary canal are produced by mitosis; growth of muscle or bone in animals, and root, leaf stem or fruit in plants, results from mitotic cell divisions. The various processes of mitosis are shown below, with fully detailed diagrams:
An exception to this occurs in the final stages of gamete production in the reproductive organs of plants and animals. The cell divisions which give rise to gametes are not mitotic but meiotic. Cells which are not involved in the production of games are called somatic cells. Mitosis takes place only in somatic cells.
When a cell is not dividing, its chromosomes become very long and thin. Along the length of the chromosomes is a series of chemical structures called genes. The chemical which forms the gene is DNA. Each gene controls some part of the chemistry of the cell. It is these genes which provide the ‘instructions’ mentioned earlier. On one chromosome there will be a gene which causes the cells of the stomach to make the enzyme pepsin. When the chromosome replicates, it builds up an exact replica of itself, gene by gene. When the chromatids separate at mitosis, each cell will receive a full set of genes. In this way, the chemical instructions in the zygote are passed on to all cells of the body. All the chromosomes, all the genes, and therefore, all the ‘instructions’ are faithfully reproduced by mitosis and passed on to complete to all the cells. Below is a diagram showing the relationship between chromosomes and genes:
Which of the ‘instructions’ are used depends on where a cell finally ends up. The gene which causes brown eyes will have no effect in a stomach cell and the gene for making pepsin will not function in the cells of the eye. So a gene’s chemical instructions are carried out only in the correct situation.
The genes which produce a specific effect in a cell are said to be expressed. In the stomach lining, the gene for pepsin is expressed. The gene for the pigment in brown eyes is not expressed.
Also, there are a fixed number of chromosomes in each species. Human body cells contain 46 chromosomes, mouse cells contain 40 and garden pea cells contain 14. The number if chromosomes in a species is the same in all of its body cells. There are 46 chromosomes in each of our liver cells, in every nerve cell skin cell and much more. Also, the chromosomes have different shapes and sizes and can be recognised by a trained observer. Also, the chromosomes are always in pairs, as our 46 chromosomes consist of 23 from our mother and 23 from our father. The number of each chromosome in each body cell of a plant or animal is called the diploid number. Because chromosomes are always found in pairs, it is always an even number.
The chromosomes of each pair are called homologous chromosomes.
Gamete production and chromosomes
The genes on the chromosomes carry the ‘instructions’ which turn a single cell zygote into an organism. The zygote is formed at fertilisation, when a male gamete fuses with a female gamete. Each gamete brings a set of chromosomes to the zygote. The gametes must then each contain only half the diploid number of chromosomes, otherwise the chromosome number would double each time an organism reproduced sexually. Each human sperm cell contains 23 chromosomes and each human ovum contains 23 chromosomes. When the sperm and ovum fuse at fertilisation, the diploid number of 46 chromosomes is produced.
The process of cell division which gives rise to gametes is different from mitosis because it results in the cells containing only half the diploid number of chromosomes, otherwise known as the haploid number and the process of cell division which dives rise to gametes is called meiosis, which only takes place on reproductive organs.
Meiosis
In a cell which is going to divide and produce gametes, the diploid numbers of chromosomes shorten and thicken as in mitosis. The pairs of homologous chromosomes, such as two long ones and two short ones which will be shown below lie alongside each other and, when the nucleus divides for the first time, it is the chromosomes and not the chromatids, which are separated . This results in only half the total number of chromosomes going to each daughter cell. As shown below, the diploid number of four chromosomes is being reduced to two chromosomes prior to the first cell division. By now, each chromosome is seen to consist of two chromatids and there is a second
division of the nucleus which separates the chromatids into four distinct nuclei, as shown in the diagram below:
This gives rise to four gametes, each with the haploid number of chromosomes. In the anther of a plant, four haploid pollen grains are produced when a pollen mother cell
divides by meiosis. As a result of meiosis and fertilization, the maternal and paternal chromosomes meet in different combinations in the zygotes. Consequently, the offspring will differ from their parents and from each other in a variety of ways.
Asexually produced organisms show no such variation because they are produced by mitosis and all their cells are identical to those of their single parent.
The Structure of the gene and protein synthesis
Chromosomes consist of a protein framework, with a long DNA molecule coiled round the framework in a complicated way. It is the DNA part of the chromosome which controls the inherited characters, and it is sections of the DNA molecule which constitute the genes.
A DNA molecule is a long chain of nucleotides, a 5-carbon sugar molecule joined to a phosphate group and an organic base. In DNA the sugar is deoxyribose, and the organic base is adenine, thymine cytosine or guanine. The nucleotides are joined by their phosphate groups to form a long chain, often thousands of nucleotides along. The phosphate and sugar molecules are the same all the way down the chain but the bases may be any one of the four in which I have listed above. Below is a diagram to show this:
The sequence of bases down the length of the DNA molecule forms a code which instructs the cell to make particular proteins. Proteins are made from amino acids linked together. The type and sequence of the amino acids joined together will determine the kind of protein formed. For example, one protein molecule may start with the sequence alanine-glycine-glycine, and a different protein may start glycine-serine-alanine. It is the sequence of the bases in the DNA molecule that decides which amino acids are used and in which order they are joined. Each group of three bases stands for one amino acid.
Below is a diagram to show protein synthesis:
A gene then is a sequence of triplets of the four bases, which specifies an entire protein. Insulin is a small protein with only 51 amino acids. Most proteins are much larger than this and most genes contain a thousand or more bases. Below is a diagram of the genetic code:
The chemical reactions which take place in a cell determine what sort of cell it is and what its functions are. These chemical reactions are controlled by enzymes, which are proteins. It follows, therefore, that the genetic code of DNA, in determining which proteins, particularly enzymes, are produced in a cell, and also determines the cell’s structure and function. In this way, the genes also determine the structure and function of the whole organism.
Replication of DNA
The DNA in a chromosome consists of two chains of nucleotides held together by chemical bonds between the bases. The size of the molecules ensures that adenine always pairs with thymine, and cytosine pairs with guanine. The double strand is twisted to form a helix, as show below:
The sequence adenine-
Cytosine-guanine is part
Of the genetic code.
Before cell division can occur, the DNA of the chromosome has to replicate, i.e. make replicas of itself. To do this, enzymes make the double strands of DNA unwind and separate into two single strands, rather like undoing a zip. Nucleotides are brought to the ‘unzipped’ DNA and joined to the exposed bases with the aid of enzymes. The adenine of an arriving nucleotide always joins to the thymine of the DNA; the cytosine of a nucleotide always joins to the guanine of the DNA. Similarly, the thymine or guanine on a nucleotide will join to the adenine or cytosine, respectively, of the DNA. The pairing is always A with T, and C with G. Thus an exposed sequence of C-C-A-T-T-G-C-A on the single strand of DNA would build up a corresponding sequence of G, G, T, A, A, C, G, T nucleotides. The new nucleotides join up to form a chain attached to the exposed strand. This happens all the way along each DNA strand. Since this is happening in both
strands of DNA, the double helix is replicated and the full set of genetic instructions is passed to both daughter cells at cell division. This is shown in the diagram below:
Mutations
A mutation is a spontaneous change in a gene or a chromosome. In a gene mutation it may be that one or more genes are not replicated correctly. A chromosome mutation may result from damage to or loss of part of a chromosome during mitosis or meiosis, or even the gain of an extra chromosome as in Down’s syndrome
An abrupt change in a gene or chromosome is likely to result in a defective enzyme and will usually disrupt the complex reactions in the cells. Most mutations, therefore, are harmful to the organism. Also, only about 3% of human DNA consists of genes. The rest consists of repeated sequences of nucleotides that do not code for proteins, and is sometimes called ‘junk DNA’, but that term only means that we do not know its function. If mutations occur in these non-coding sequences, they are unlikely to have any effect on the organism and are, therefore, described a neutral. Rarely a gene or chromosome mutation produces a beneficial effect and this may contribute to the success of the organism.
All of which I have explained in a detailed manner really does show us that DNA is a rather amazing thing, and it controls our whole lives, as well as the lives of plants, and other animals. Mitosis; meiosis; protein synthesis; replication; gamete production; mutations and the structure of DNA itself are altogether rather extraordinary.
