2 DNA the blueprint of life

Transcription

1 Biology Form 5 Page 17 Ms. R. Buttigieg 2 DNA the blueprint of life See GCSE Biology Chapter Genetics and heredity 2.1 The D A molecule. DNA stands for deoxyribonucleic acid. It was first isolated by Johann Friedrich Miescher in Its structure was described by James Watson and Francis Crick in the 1950's. They described DNA molecule as being a twisted ladder, or double helix. ucleic Acids - are large, complex molecules made from smaller molecules called nucleotides (just like proteins are made from amino acids). Apart from DNA, an example of another nucleic acid is RNA ribose nucleic acid. Each nucleotide is made of: A sugar A nitrogenous base A phosphate group Several nucleotides can join to form polynucleotides or nucleic acids In DNA we find four 4 different nucleotides due to 4 different nitrogenous bases: Adenine (A) Thymine (T) Cytosine (C) Guanine (G) These always pair in the following way Adenine + Thymine Cytosine + Guanine These bases join together by hydrogen bonds and form the rungs of the twisted ladder. The phosphates and sugar deoxyribose form the backbone.

2 Biology Form 5 Page 18 Ms. R. Buttigieg So, the DNA molecule is made up of a number of smaller molecules called nucleotides. These nucleotides in turn combine to form nucleic acids. The phosphate group of one nucleotide joins to the sugar of the next. The bases attached to the sugar groups stick out from the strand. In DNA, two strands are bonded together to form a double helix, which is made up of 2 intertwined strands. DNA is found in the chromosomes of every cell. These can be seen only during cell division due to the heavy coiling of chromosomes. (from The role of D A in protein synthesis. The sequence of bases on the DNA strand determines the sequence of amino acids and therefore the type of protein (i.e. characteristics) that will be produced.

3 Biology Form 5 Page 19 Ms. R. Buttigieg In a DNA strand, every 3 bases on the chain, code for a particular amino acid. This is known as the triplet code of D A. The kind of proteins produced in the organism depend on the sequence of bases on the DNA molecule. A gene is a sequence of nucleotides on the DNA molecule, which code for an entire protein. The longer the protein, the longer the gene. A gene is as a section of D A controlling an identifiable characteristic e.g. eye colour Structure and function of chromosomes Along the length of chromosomes there are many structures called genes Genes consist of a strand of DNA which codes for an entire protein The genes provide the instructions for the cell e.g. a gene may instruct the stomach to produce the enzyme pepsin. A chromosome is made of 2 chromatids joined at the centromere Different organisms contain a different number of chromosomes. This number is the same in all body cells except in the gametes (sex cells) Chromosomes occur in pairs having similar shape and size - homologous Humans have 46. o Body cells all have a diploid (2n) number of chromosomes 46 in our case o Gametes have a haploid (n) number of chromosomes half the number 23 in our case

4 Biology Form 5 Page 20 Ms. R. Buttigieg Alleles as alternative forms of a gene. A human egg cell and a human sperm cell both contain 23 single chromosomes. Fertilisation brings these two sets of single chromosomes together to make 23 pairs of chromosomes in the embryo. Each of these pairs of chromosomes contains genes inherited from the father and genes inherited from the mother, and these genes are in pairs, both coding for the same characteristic - so you have two genes controlling eye colour, for example. These different forms of the same gene are called alleles (pronounced al-eel). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. If both the alleles for a particular characteristic are the same they are called homozygous. If they are different from each other they are called heterozygous. Alleles may be either dominant or recessive. A dominant allele always shows. A recessive allele only shows when it is on its own (because it's on a sex chromosome) or when both the alleles are the same (homozygous). Summary

5 Biology Form 5 Page 21 Ms. R. Buttigieg

6 Biology Form 5 Page 22 Ms. R. Buttigieg Mutations can occur on a chromosome or on a gene level. Gene mutation Can result when one base is changed with another e.g. thymine instead of cytosine. A gene controls production of the skin pigment melanin. This protects the skin from UV light. In albinos, the structure of the gene changes or mutates. So it no longer codes for the production of melanin. Cystic fibrosis is also a disease caused by a gene mutation. Chromosome mutation

7 Biology Form 5 Page 23 Ms. R. Buttigieg 2.2 Cell Division This is important for: The body to grow Passing chromosomes from one cell to another There are 2 main types of cell division: 1. Meiosis occurs in the reproductive organs. 2. Mitosis occurs in all the other cells (somatic cells) See GCSE Biology Chapter 21 Mitosis occurs in: Growth Asexual reproduction e.g. in the Amoeba For a more detailed explanation see GCSE Biology pg. 183, figure 21.2 For a more detailed explanation of Meiosis see GCSE Biology pg. 185, figure 21.7

8 Biology Form 5 Page 24 Ms. R. Buttigieg

9 Biology Form 5 Page 25 Ms. R. Buttigieg See also the Table on GCSE Biology pg. 186 Mitosis One division Two (2) daughter cells per cycle Daughter cells genetically identical Called clones Same chromosome no. as parents Occurs in somatic cells Throughout life cycle Used in growth, repair, asexual reproduction Meiosis Two divisions Four (4) daughter cells per cycle Daughter cells genetically different Chromosome no. half that of parents Occurs in reproductive cells Completed after sexual maturity Sexual reproduction, new gene combinations Work out GCSE Biology pg. 186 number Chromosomes occur in pairs in all cells except the gametes. a) What are alleles? b) Why are there 2 alleles for each character? c) Explain what is meant by dominant and recessive alleles. 2. Red/green colour blindness is sex-linked and caused by a recessive allele. One in 80 males are colour blind, but only 1 in 6400 females. a) On which chromosome is the allele for colour blindness found? b) Explain why this condition occurs far more often in males than in females.

10 Biology Form 5 Page 26 Ms. R. Buttigieg 2.3 Variation In asexual reproduction there is no variation as clones (identical cells) are produced by mitosis. In sexual reproduction we get new gene combinations in individuals so all offspring are different and have a unique genetic set. When the sperms (haploid 23 chromosomes) and the ova (haploid 23 chromosomes in each nucleus) join together we say that fusion of gametes has occurred to give the diploid chromosome number of 46. This variation results from two important processes known as independent assortment and random variation. Independent assortment This occurs when during meiosis half the chromosomes go to one gamete and half to the other gamete. These go randomly so for example a gamete may get: 5 maternal and 18 paternal chromosomes 12 maternal and 11 paternal chromosomes. Random variation The sperm fertilising the ova does so randomly. Sometimes harmful genes present in some sperms or ova may never get fertilized and they are lost and not expressed in the offspring. So in this case the variation is not inherited. As a result of both independent assortment and random fertilization each individual has a different set of genes and this gives rise to variation even within a species. Draw a diagram to show the structure of the D A molecule. What does a nucleotide consist of? Define: chromosome, gene, chromatid, centromere, homologous chromosomes, diploid cell.

11 Biology Form 5 Page 27 Ms. R. Buttigieg Continuous and discontinuous variation Variation, the small differences that exist between individuals, can be described as either continuous or discontinuous. These differences can be due to the environment (non-inherited) or heredity. CO TI UOUS VARIATIO - In continuous variation there is a complete range of measurements from one extreme to the other. Other examples include: Weight Hand span Length of feet Milk yield in cows Continuous variation is the combined effect of many genes and is significantly affected by environmental influences. Milk yield in cows, for example, is determined not only by breeding programs but is also significantly affected by environmental factors such as pasture quality and diet, weather, and the comfort of their surroundings. DISCO TI UOUS VARIATIO - This is where individuals fall into a number of distinct classes or categories, and is based on features that cannot be measured across a complete range. You either have the characteristic or you don't. Blood group is a good example: you are either one blood group or another - you can't be in between. Another example is whether you are a male or a female. Discontinuous variation is controlled by alleles of a single gene or a small number of genes. Continuous variation Continuous range of values Influenced mainly by environment Usually controlled by many genes E.g. height, weight, intelligence, skin colour Discontinuous variation Distinct values without intermediates Influenced mainly by genes Controlled by one/two genes E.g. blood group, eye colour, ability to roll tongues, sex

12 Biology Form 5 Page 28 Ms. R. Buttigieg Choose the correct answer from the following 1. Which part of a cell carries the information that is passed from one generation to the next? A. Cell B. Nucleus C. Cytoplasm D. Chloroplast 2. Why are identical twins different from non-identical twins? A. They look the same B. They look different C. They have the same genes D. They have the same eggs 3. Which of the following is the word used to describe what happens when the nucleus of a sperm joins with the nucleus of an egg cell? A. Sex B. Fertilization C. Intercourse D. Reproduction 4. Which of the following statements is true about the variation? A. Variation is caused by genes B. Variation is caused by the environment C. Variation can caused by both genes and the environment D. None of the above 5. There are two types of variation. These are continuous and discontinuous variation. Which of the following statements is false? A. Continuous variation produces a spread of variation within a population B. Discontinuous variation is the result of genetics alone C. Blood group is an example of discontinuous variation D. Discontinuous variation is influenced by the environment 6. Some types of variation are due to changes in the genetic material. What is this type of change called? A. Fertilization B. Mutation C. Radiation D. Sterilization 7. Some people can roll their tongues whilst others cannot. What is this an example of? A. Genetically inherited characteristic B. Continuous variation C. Adaptation D. Specialization 8. Which type of variation can be presented as a line graph? A. Continuous B. Genetic C. Discontinuous D. Adapted

13 Biology Form 5 Page 29 Ms. R. Buttigieg Answer these on your A4 notebook 1. In table form give 3 differences between mitosis and meiosis. 2. Give 1 similarity between mitosis and meiosis. 3. Explain (in a paragraph) why meiosis is important in sexual reproduction 4. What are mutations? Distinguish between chromosome and gene mutations and give an example of each. 5. What are mutagens? Give an example. 6. Give 2 sources of variation in sexual reproduction. 7. Distinguish between continuous and discontinuous variation and give an example of each. 2.4 Mendelian Genetics & the laws of inheritance Some definitions you must be sure of: 1. Genotype The genes that an organism possesses. The set of alleles inherited from both parents. 2. Phenotype The individual s observed appearance. 3. Dominant allele Always expressed. Shown by a capital letter e.g. B 4. Recessive allele Not expressed in the phenotype when present with a dominant allele of the same type. Shown by a small letter e.g. b 5. Homozygous genotype the individual has inherited two identical alleles e.g. BB (homozygous dominant) or bb (homozygous recessive) 6. Heterozygous genotype the individuals has inherited two different alleles e.g. Bb Genetics is the study of heredity and variation in organisms. We begin with a study of the monohybrid cross, invented by Mendel. In a monohybrid cross, organisms differing in only one trait are crossed. Our objective is to understand the principles that govern inheritance in plants and animals, including humans, by solving problems related to the monohybrid cross.

14 Biology Form 5 Page 30 Ms. R. Buttigieg

15 Biology Form 5 Page 31 Ms. R. Buttigieg First draw a Punnett square then fill it in to see the available possibilities Give the percentages for possible genotypes and phenotypes Child Phenotypes Brown % Blue % Child Genotypes

16 Biology Form 5 Page 32 Ms. R. Buttigieg Work out the following: a. Using the symbol W = normal wing and w = short wing: i. The genotype of a fly which is heterozygous for this character ii. The possible genotypes of its gametes iii. Work out what kind of offspring would be produced if a heterozygous fly mated with one which was homozygous for normal wing. b. A red flowered tulip was crossed with a white-flowered tulip. The seeds were collected and all grew into red tulips. i. Which colour was dominant? ii. What was the genotype of the offspring? iii. Some of the offspring were self-fertilised. The plants produced had 3 times as many red as white flowers. Use a Punnett square to explain this cross. c. A Roman nose is dominant to a straight nose. A man with a Roman nose marries a woman with a straight nose. The man s mother also had a straight nose. Calculate the chance of one of their children also having a straight nose.

17 Biology Form 5 Page 33 Ms. R. Buttigieg Doing a recessive backcross Red flowered plants can have 2 possible genotypes RR or Rr. So when you see a red flower how can you tell which is its genotype? One way of finding out is by crossing these red flowers with the original white flowered homozygous recessive plants (rr). This is called a recessive back cross. If the red flowered plant is RR, then all the offspring should be red (Rr). If however the red flowered plant is Rr, then we will get a mixture of both red-flowered (Rr) and white flowered plants (rr) White (rr) x Red (RR) White (rr) x Red (Rr) Breeding True Individuals which have the same alleles for a characteristics e.g. NN or nn are said to breed true. When you cross two pure breeds which are different e.g. NN with nn you get hybrids. Hybrids do not breed true. Pedigrees the tracing of a particular characteristic in an individual s ancestry. Some features persist in families, for from a pedigree you can work out the genotypes of the various individuals. A pedigree chart is a chart which tells you all of the known phenotypes for an organism and its ancestors, most commonly humans, show dogs, and race horses.

18 Biology Form 5 Page 34 Ms. R. Buttigieg Codominance - Situation in which two different alleles for a genetic trait are both expressed. An example of this is people who have an AB blood type for the ABO blood system. When they are tested, these individuals actually have the characteristics of both type A and type B blood. Their phenotype is not intermediate between the two. A and B are equally dominant to each other, while O is recessive to both of them. When A and B are present together in a genotype, both will be expressed to effect the phenotype. The symbols used are: I A allele A, I B allele B, i allele O Suppose a man who is heterozygous for blood group A marries a woman who is heterozygous for blood group B. Give the ratio of the genotype of their children. Other examples of codominance include coat colour in cattle (red/white/roan), and coat colour in cats (black/orange/tortoiseshell). Another example of codominance is sickle cell haemoglobin in humans. The gene for haemoglobin Hb has two codominant alleles: Hb A (the normal gene) and Hb S (the mutated gene). There are three phenotypes: Hb A Hb A Hb A Hb S Hb S Hb S Normal. All haemoglobin is normal, with normal red blood cells. Sickle cell trait. 50% of the haemoglobin in every red blood cell is normal, and 50% is abnormal. The red blood cells are slightly distorted, but can carry oxygen, so this condition is viable. However these red blood cells cannot support the malaria parasite, so this phenotype confers immunity to malaria. Sickle cell anaemia. All haemoglobin is abnormal, and molecules stick together to form chains, distorting the red blood cells into sickle shapes. These sickle red blood cells are destroyed by the spleen, so this phenotype is fatal.

19 Biology Form 5 Page 35 Ms. R. Buttigieg Work these on your A4 notebook. 1. A heterozygous woman of blood group A marries a man of blood group O. Carry out a cross to find the genotype of their children. 2. A man and a woman both having the sickle cell trait have a child. Carry out a cross to determine the genotype of this child. What chance of surviving does this child have? Sex determination and sex-linkage Sex Determination Sex is determined by the sex chromosomes (X and Y). In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY), though in other species it is the other way round. The inheritance of the X and Y chromosomes can be demonstrated using a monohybrid cross: This shows that there will always be a 1:1 ratio of males to females. Note that female gametes (eggs) always contain a single X chromosome, while the male gametes (sperm) can contain a single X or a single Y chromosome. Sex is therefore determined solely by the sperm and in the testes, an approximately equal number of X and Y sperms is produced. There are techniques for separating X and Y sperm, and this is used for planned sex determination in farm animals using IVF. ( Sex-linkage Certain harmful recessive alleles e.g. red-green colour blindness and haemophilia are carried on the X chromosome. Therefore if a woman inherits a harmful allele on one of her X chromosomes, it will be masked by the dominant allele on the other X chromosome. The woman is said to be a carrier, but is not herself affected by it. She can however pass it on to her children.

20 Biology Form 5 Page 36 Ms. R. Buttigieg If a man inherits a defective allele on his X chromosome he will be suffering from the disease as it cannot be masked due to the short Y chromosome. A male can never be a carrier of such sex-linked diseases. So such diseases are more common in males than females. A well-known example of a sex-linked characteristic is colour blindness in humans. 8% of males are colour blind, but only 0.7% of females. The genes for green-sensitive and red-sensitive rhodopsin are on the X chromosome, and mutations in either of these lead to colour blindness. The diagram below shows two crosses involving colour blindness, using the symbols X R for the dominant allele (normal rhodopsin, normal vision) and X r for the recessive allele (non-functional rhodopsin, colour blind vision). Normal female - X R X R ; Carrier female - X R X r ; Normal male - X R Y; Colour blind male X r Y Show the possibilities when (a) a carrier female marries a normal male (b) a normal female marries a colour blind male (c) a carrier female marries a colour blind male. Other examples of sex linkage include haemophilia, premature balding and muscular dystrophy. On your A4 notebook work out this problem: A man sick with haemophilia marries a normal woman, who is not a carrier. Carry out the cross between these two to show the genotype of their children.

21 Biology Form 5 Page 37 Ms. R. Buttigieg Genetic problems 1. The human lung condition cystic fibrosis is controlled by a pair of alleles. Only those who are homozygous for the recessive allele suffer from the disease. Heterozygous individuals don t suffer but are carriers. i. Using the symbol N for the dominant allele and n for the recessive one, write the genotypes of (a) the carrier and (b) someone who suffers from the disease. ii. What percentage of children from parents who are both carriers, will be likely to be carriers? 2. In sheep, black wool is caused by a recessive allele b and white wool by a dominant allele B. In a flock of sheep there were some black males and a lot of heterozygous females. i. Give the genotype of the black male sheep ii. Give the genotype of the females. iii. What percentage of the young would you expect to have black wool? iv. Explain how you could find out if the genotype of a sheep with white wool was homozygous or heterozygous. 3. The flower colour of a plant is controlled by a pair of alleles. When a red-flowered plant was pollinated with pollen from a white flowered plant, the resulting offspring all had pink flowers. Use the symbol C R for the allele controlling red colour and C W for the allele controlling white colour. i. What is the genotype of the white flowered parent plant? ii. Give the genotype of the pink flowers. iii. Give the genotypes of the young produced by crossing a red flowered parent plant with a pink flowered plant. Give the ratio. 4. A homozygous brown-eyed man marries a homozygous blueeyed woman. What will be the percentage of the genotypes and phenotypes of their children?

22 Biology Form 5 Page 38 Ms. R. Buttigieg Genetics JL Exam Questions 1. Read the following paragraph and answer the questions below: When the mechanism of inheritance of flower colour in garden peas was investigated, red-flowered plants were crossed with white-flowered plants. The first generation of plants all had red flowers. However, when these red-flowered plants were allowed to self-fertilise, about 25% of the offspring had white flowers, the remainder having red flowers. In a similar investigation with snapdragon plants, when red-flowered plants were crossed with white-flowered plants, the resulting first generation all had pink flowers. When these pink-flowered plants were self-fertilised, 25% of the offspring had white flowers, 25% had red flowers and 50% had pink flowers. a. Suggest why the results obtained with the garden pea are different from those with the snapdragon plants. (2) b. Using symbols and a written explanation, account fully for the result obtained with garden peas. (4) c. What would be the results of interbreeding the white garden peas. (2) d. Draw a genetic diagram, to show how the results for the snapdragon can be explained genetically. (3) e. Write down the genotype of the snapdragon plants with red flowers. (1) f. Describe an experiment which you could perform to establish the genotype of the garden peas plants with red flowers. (3) (15 marks) 2. a. Draw and label a large diagram to show the structure of a named insect pollinated flower you have studied. (5) b. State the functions of 2 parts of the flower that you have labelled. (2) c. Suggest two (2) ways by which an insect-pollinated flower might differ from a wind pollinated flower. (2) d. The flower colour of a certain species of plant is controlled by a single pair of alleles. When a red-flowered plant was pollinated with pollen from a white-flowered plant, the resulting offspring all had pink flowers.

23 Biology Form 5 Page 39 Ms. R. Buttigieg Use the symbol R for allele controlling red colour and W for the allele controlling white colour. (i) Give the genotype of the white-flowered parent plant.(1) (ii) Give the genotype of the pink-flowered plant. (1) (iii) Give the genotypes of the progeny/offspring produced by crossing the red flowered parent plant with one of the pink-flowered plants and the ratio in which they appear. (2) (iv) Give the genotypes of the progeny/offspring produced by self-pollination of two of the pinkflowered plants and the ratio in which they appear. (2) (total 15 marks) 3. The family tree below shows the pattern of inheritance of a genetic disorder. a) Is the disease controlled by a dominant or a recessive allele ( or n)? (1) b) Give a reason to support your answer to a. (1) c) It is unlikely that the disease being referred to in the above diagram is sex linked. Give a reason. (1) d) Name a sex linked disease. (1) e) What must be the genotype of: i) Elaine ii) her mother iii) her father (3) f) What might Simon s genotype be? Give a reason. (2) Total 9 marks