Section E: Variation and Selection

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1 hapter 16: hromosomes, enes and DN his chapter looks at the structure and organisation of genetic material, namely chromosomes, genes and DN. Section E: Variation and Selection body (made of cells) chromosome (contains DN) cell nucleus (contains chromosomes) DN DN is short for deoxyribonucleic acid. It gets its deoxyribo name from the sugar in the DN molecule. his is deoxyribose a sugar containing five carbon atoms. hapter 16: hromosomes, enes and DN Figure 16.1 Our genetic make-up. Fig_1601_ he chemical that is the basis of inheritance in nearly all organisms is DN. DN is usually found in the nucleus of a cell, in the chromosomes (Figure 16.1). small section of DN that determines a particular feature is called a gene. enes determine features by instructing cells to produce particular proteins which then lead to the development of the feature. So a gene can also be described as a section of DN that codes for a particular protein. DN can replicate (make an exact copy of) itself. When a cell divides by mitosis (see hapter 17), each new cell receives exactly the same type and amount of DN. he cells formed are genetically identical. Figure 16.2 (a) Watson and rick with their doublehelix model. he structure of DN Who discovered it? James Watson and Francis rick, working at ambridge University, discovered the structure of the DN molecule in 1953 (Figure 16.2a). Both were awarded the Nobel prize in 1962 for their achievement. However, the story of the first discovery of the structure of DN goes back much further. Watson and rick were only able to propose the structure of DN because of the work of others Rosalind Franklin (Figure 16.2b) had been researching the structure of a number of substances using a technique called X-ray diffraction. (b) Rosalind Franklin ( ). 181

2 hapter 16: hromosomes, enes and DN S cytosine is always opposite guanine adenine is always opposite thymine S S S S S S S hydrogen bonds hold the pairs of bases together phosphate groups hold the nucleotides in each strand together Key phosphate S deoxyribose sugar adenine thymine guanine cytosine Figure 16.4 art of a molecule of DN Not only is the DN code universal, but the actual DN in different organisms is very similar. 98% of our DN is the same as that of a chimpanzee; 50% of it is the same as that of a banana! Watson and rick were able to use her results, together with other material, to propose the now-familiar double helix structure for DN. Rosalind Franklin died of cancer and so was unable to share in the award of the Nobel rize (it cannot be awarded posthumously). molecule of DN is made from two strands of nucleotides, making it a polynucleotide. Each nucleotide contains a nitrogenous base (adenine (), thymine (), cytosine () or guanine ()), a sugar molecule and a phosphate group (Figure 16.3). phosphate group sugar molecule Figure 16.3 he structure of a single nucleotide. one entire gene nitrogenous base (adenine, thymine, cytosine or guanine) Notice that, in the two strands (see Figure 16.4), nucleotides with adenine are always opposite nucleotides with thymine, and cytosine is always opposite guanine. denine and thymine are complementary bases, as are cytosine and guanine. omplementary bases always bind with each other and never with any other base. his is known as the base-pairing rule. DN is the only chemical that can replicate itself exactly. Because of this, it is able to pass genetic information from one generation to the next as a genetic code. he DN code Only one of the strands of a DN molecule actually codes for the manufacture of proteins in a cell. his strand is called the sense strand. he other strand is called the anti-sense strand. he proteins manufactured can be intracellular enzymes (enzymes that control processes within the cell), extracellular enzymes (enzymes that are secreted from the cell to have their effect outside the cell), structural proteins (e.g. used to make hair, haemoglobin, muscles, cell membranes) or hormones. roteins are made of chains of amino acids. sequence of three nucleotides in the sense strand of DN codes for one amino acid. s the sugar and phosphate are the same in all nucleotides, it is actually the bases that code for the amino acid. For example, the base sequence codes for the amino acid cysteine. Because three bases are needed to code for one amino acid, the DN code is a triplet code. he sequence of bases that codes for all the amino acids in a protein is a gene (Figure 16.5). DN base sequences amino acids coded for arginine isoleucine proline leucine phenylalanine Figure 16.5 he triplet code. one entire protein he triplets of bases that code for individual amino acids are the same in all organisms. he base sequence codes for the amino acid cysteine in humans, bacteria, bananas, monkfish, or in any other organism you can think of the DN code is a universal code. 182

3 DN replication When a cell is about to divide (see Mitosis, hapter 17) it must first make an exact copy of each DN molecule in the nucleus. his process is called replication. s a result, each cell formed receives exactly the same amount and type of DN. Figure 16.6 summarises this process. One consequence of the base-pairing rule is that, in each molecule of DN, the amounts of adenine and thymine are equal, as are the amounts of cytosine and guanine. 1 he polynucleotide 2 Each strand acts as 4 strands of DN a template for the formation separate. of a new strand of DN. Figure 16.6 How DN replicates itself. DN polymerase assembles nucleotides into two new strands according to the base-pairing rule. ene mutations when DN makes mistakes mutation is a change in the DN of a cell. It can happen in individual genes or in whole chromosomes. Sometimes, when DN is replicating, mistakes are made and the wrong nucleotide is used. he result is a gene mutation and it can alter the sequence of the bases in a gene. In turn, this can lead to the gene coding for the wrong protein. here are several ways in which gene mutations can occur (Figure 16.7). In duplication, Figure 16.7 (a), the nucleotide is inserted twice instead of once. Notice that the entire base sequence is altered each triplet after the point where the mutation occurs is changed. he whole gene is different and will now code for an entirely different protein. In deletion, Figure 16.7 (b), a nucleotide is missed out. gain, the entire base sequence is altered. Each triplet after the mutation is changed and the whole gene is different. gain, it will code for an entirely different protein. In substitution, Figure 16.7 (c), a different nucleotide is used. he triplet of bases in which the mutation occurs is changed and it may code for a different amino acid. If it does, the structure of the protein molecule will be different. his may be enough to produce a significant alteration in the functioning of a protein or a total lack of function. However, the new triplet may not code for a different amino acid as most amino acids have more than one code. In inversions, Figure 16.7 (d), the sequence of the bases in a triplet is reversed. he effects are similar to substitution. Only one triplet is affected and this may or may not result in a different amino acid and altered protein stucture. 3 wo identical DN molecules are formed each contains a strand from the parent DN and a new complementary strand. (a) (b) (c) (d) duplication here extra becomes first base of next triplet deletion here original base substituted base replaced by first base of next triplet inversion here Figure 16.7 ene mutations (a) duplication, (b) deletion, (c) substitution, (d) inversion. hapter 16: hromosomes, enes and DN 183

4 Mutations that occur in body cells, such as those in the heart, intestines or skin, will only affect that particular cell. If they are very harmful, the cell will die and the mutation will be lost. If they do not affect the functioning of the cell in a major way, the cell may not die. If the cell then divides, a group of cells containing the mutant gene is formed. When the organism dies, however, the mutation is lost with it; it is not passed to the offspring. Only mutations in the sex cells or in the cells that divide to form sex cells can be passed on to the next generation. his is how genetic diseases begin. hapter 16: hromosomes, enes and DN Sometimes a gene mutation can be advantageous to an individual. For example, as a result of random mutations, bacteria can become resistant to antibiotics. Resistant bacteria obviously have an advantage over non-resistant types if an antibiotic is being used. hey will survive the antibiotic treatment and reproduce. ll their offspring will be resistant and so the proportion of resistant types in the population of bacteria will increase as this happens in each generation. his is an example of natural selection (see hapter 19). ests can become resistant to pesticides in a similar way. ene mutations are random events that occur in all organisms. he rate at which they occur can be increased by a number of agents called mutagens. Mutagens include: ionising radiation (such as ultraviolet light, X-rays and gamma rays) chemicals including mustard gas and nitrous oxide, many of the chemicals in cigarette smoke and the tar from cigarettes, and some of the chemicals formed when food is charred in cooking. he structure of chromosomes Each chromosome contains one double-stranded DN molecule. he DN is folded and coiled so that it can be packed into a small space. he DN is coiled around proteins called histones (Figure 16.8). supercoiled DN/histones chromosome DN coiled around histones histones DN Figure 16.8 he structure of a chromosome. Fig_1610_ 184

5 Because a chromosome contains a particular DN molecule, it will also contain the genes that make up that DN molecule. nother chromosome will contain a different DN molecule, and so will contain different genes. How many chromosomes? Nearly all human cells contain 46 chromosomes. he photographs in Figure 16.9 show the 46 chromosomes from the body cells of a human male and female. (a) (b) Figure 16.9 hromosomes of a human male (a) and female (b). picture of all the chromosomes in a cell is called a karyotype. he chromosomes are not arranged like this in the cell. he original photograph has been cut up and chromosomes of the same size and shape paired up. he cell from the male has 22 pairs of chromosomes and two that do not form a pair the X and Y chromosomes. body cell from a female has 23 matching pairs including a pair of X chromosomes. airs of matching chromosomes are called homologous pairs. hey carry genes for the same features in the same sequence (Figure 16.10). ells with chromosomes in pairs like this are diploid cells. Not all human cells have 46 chromosomes. Red blood cells have no nucleus and so have none. Sex cells have only 23 just half the number of other cells. hey are formed by a cell division called meiosis (see hapter 17). Each cell formed has one chromosome from each homologous pair, and one of the sex chromosomes. ells with only half the normal diploid number of chromosomes, and therefore only half the DN content of other cells, are haploid cells. When two sex cells fuse in fertilisation, the two nuclei join to form a single diploid cell (a zygote). his cell has, once again, all its chromosomes in homologous pairs and two copies of every gene. It has the normal DN content. enes and alleles enes are sections of DN that control the production of proteins in a cell. Each protein contributes towards a particular body feature. Sometimes the feature is visible, such as eye colour or skin pigmentation. Sometimes the feature is not visible, such as the type of haemoglobin in red blood cells or the type of blood group antigen on the red blood cells. Red blood cells have no nucleus, therefore no chromosomes. his gives them more room for carrying oxygen. he X and the Y chromosomes are the sex chromosomes. hey determine whether a person is male or female. a homologous pair of chromosomes gene gene B gene genes, B, and each control a different feature Figure Both chromosomes in a homologous pair have the same sequence of genes. hapter 16: hromosomes, enes and DN 185

6 Some genes have more than one form. For example, the genes controlling several facial features have alternate forms, which result in alternate forms of the feature (Figure 16.11). Form 1 of the gene Form 2 of the gene earlobe attachment hapter 16: hromosomes, enes and DN 186 a homologous pair of chromosomes B c Figure and a, B and b, and c are different alleles of the same gene. hey control the same feature but code for different expressions of that feature. a B free earlobe upper eyelid not folded eyes angled away from nose attached earlobe upper eyelid folded eyes angled towards nose long eyelashes short eyelashes Figure he alternate forms of four facial features. upper eyelid fold angle of eyes to nose length of eyelashes he gene for earlobe attachment has the forms attached earlobe and free earlobe. hese different forms of the gene are called alleles. Homologous chromosomes carry genes for the same features in the same sequence, but the alleles of the genes may not be the same (Figure 16.12). he DN in the two chromosomes is not quite identical. Each cell with two copies of a chromosome also has two copies of the genes on those chromosomes. Suppose that, for the gene controlling earlobe attachment, a person has one allele for attached earlobes and one for free earlobes. What happens? Is one ear free and the other attached? re they both partly attached? Neither. In this case, both earlobes are free. he free allele is dominant and switches off the attached allele, which is recessive. See hapter 18 for more detail on how genes are inherited. hromosome mutations When cells divide, they do not always divide properly. Bits of chromosomes can sometimes break off one chromosome and become attached to another. Sometimes one daughter cell ends up with both chromosomes of a homologous pair whilst the other has none. hese mistakes are called chromosome mutations and usually result in the death of the cells formed. Sometimes sex cells do not form properly and they contain more (or fewer) chromosomes than normal. One relatively common chromosome mutation results in ova (female sex cells) containing two copies of chromosome 21. When an ovum like this is fertilised by a normal sperm, the zygote will have three copies of chromosome 21. his is called trisomy (three copies) of chromosome 21. Unlike some other chromosome mutations, the effects of this mutation are usually nonfatal and the condition that results is Down s syndrome (Figure 16.13).

7 Down s syndrome children sometimes die in infancy, as heart and lung defects are relatively common. hose that survive have a near normal life span. Individuals with Down s syndrome can now live much more normal lives than was thought possible just 20 years ago. hey require much care and attention during childhood, and particularly in adolescence, but, given this care, they can achieve good social and intellectual growth. Most importantly, they achieve personal self-sufficiency. risomy of chromosome 21 is more common in women over 40 years of age. s a result, they have more babies with Down s syndrome than younger women. Figure his boy has Down s syndrome. His teacher is helping him to develop his full potential. hapter 16: hromosomes, enes and DN 187

8 End of hapter hecklist You should now be able to: hapter 16: hromosomes, enes and DN 188 E recall that the nucleus of a cell contains chromosomes on which genes are located understand that a gene is a section of a molecule of DN describe the structure of a DN molecule understand, in outline, how DN acts as a genetic code understand, in outline, how DN is replicated recall that in human cells the diploid number of chromosomes is 46 and the haploid number is 23 understand the meaning of alleles of a gene (see also hapter 18) recall that mutation is a rare, random change in genetic material that can be inherited understand that many mutations are harmful but some are neutral and a few are beneficial (see also hapter 19) understand how resistance to antibiotics can increase in bacterial populations understand that the incidence of mutations can be increased by mutagens such as ionising radiation and some chemicals. Questions More questions on DN can be found at the end of Section E on page he diagram represents part of a molecule of DN. B D a) Name the parts labelled, B,, D and E. b) What parts did James Watson, Frances rick and Rosalind Franklin play in discovering the structure of DN? c) Use the diagram to explain the base-pairing rule. 2 a) What is: i) a gene ii) an allele? b) Describe the structure of a chromosome. c) How are the chromosomes in a woman s skin cells: i) similar to ii) different from those in a man s skin cells? 3 DN is the only molecule capable of replicating itself. Sometimes mutations occur during replication. a) Describe how DN replicates itself. b) Explain how a single gene mutation can lead to the formation of a protein in which: i) many of the amino acids are different from those coded for by the non-mutated gene ii) only one amino acid is different from those coded for by the non-mutated gene.

9 4 he graph shows the numbers and relative frequency of births of Down s syndrome babies in women aged between 20 and 50. Key number of Down s syndrome babies born Down s syndrome births as a percentage of all births number of Down s syndrome babies born per year mother s age (years) a) What is Down s syndrome? b) How do the numbers of Down s syndrome births change with the age of the mother? Down s syndrome births as a percentage of all births c) Suggest why the trend shown by the frequency of Down s syndrome births is different from that shown by the actual numbers. hapter 16: hromosomes, enes and DN 189

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