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2.5 GENETICS 2.5.2 Heredity and Gene Expression Definition of “heredity”. Heredity is the passing on of features from parents to offspring by means of genes. Heredity is also called genetic inheritance. The features are passed from the parents to the offspring by means of genes in the sex cells or gametes. Genes are the units of heredity. Example of “heredity”. Humans inherit genes that control features like eye colour and ear shape. Plants inherit genes that control features like flower colour and seed shape. Definition and example of “gene expression”. Genes are located along the length of a chromosome. Each gene is a section of DNA and contains the code for making a particular protein. Each gene or group of genes controls the development of a particular characteristic or trait of an individual. Gene expression is the way in which the genetic code in a gene is used to make a protein Basically gene expression refers to the way in which genes work. Gene expression depends on the environment which means that characteristics result from heredity and environment. A child might inherit the gene for tallness but if it doesn’t get enough food the genes will not work and the child will not grow tall i.e. the genes will not be expressed A plant inherits the genes to make chlorophyll but if it doesn’t get enough light the genes will not work and no chlorophyll will be make i.e. the genes will not be expressed. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS 2.5.3 Genetic Code Definition of a “gene”. Genes are located along the length of a chromosome. Each gene is a section of DNA and contains the code for making a particular protein. Each gene or group of genes controls the development of a particular characteristic or trait of an individual. A gene is inherited information for making a protein A gene is a section of DNA that causes the production of a specific protein Role of a “gene”. Many of the proteins made by genes are enzymes. Genes control a cell because the enzymes they make control cell activities. Therefore, the role of a gene is to control a cell. Chromosome structure. Chromosomes are made of DNA (40%) and protein (60%). The DNA is a very long molecule. It is very coiled and folded. The proteins called histones are responsible for holding the DNA in its folded state so that it can fit into the nucleus of a cell. Genes are found on the DNA. Some genes are close together and others are far apart. There are many parts of the DNA that contains no genes. This DNA is said to be non-coding and is “junk DNA”. 2.5.4 DNA Structure, Replication and Profiling Simple structure of DNA DNA is a very long molecule. It consists of two strands. The two strands are linked together by paired bases. There are four different bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) Each base can only link with one other type, A with T and C with G. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS Coding and non-coding structures. Genes are arranged along the DNA. Sometimes the genes are close together and sometimes they are widely separated. The gene sections of DNA are known as coding structures. The rest of the DNA is non-coding, as it doesn’t carry the code for the formation of a protein. This is often called junk DNA. There are 2 types of junk DNA a. Non-coding DNA ( found between genes) This DNA consists of repeating sequences of bases dispersed at random between genes. The function of these repeating stretches of bases is unknown. b.Non-coding DNA (found within genes) The sections of DNA that code for a protein a gene are called exons (because they are expressed). The non-coding sections of DNA in a gene are called introns (because they intervene between the working sections). Triplet base code. DNA codes for a protein by using a sequence of three consecutive bases. Each group of three bases is called a triplet or codon. Each triplet codes for an amino acid. A sequence of triplets codes for a sequence of amino acids and hence a protein will be formed. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS H.2.5.14 NUCLEIC ACID STRUCTURE AND FUNCTION (EXTENDED STUDY) Structure of DNA – a nucleic acid 1. DNA is a very long double-stranded molecule. 2. It is composed of NUCLEOTIDES joined together (it is a polymer). A nucleotide consists of ONE DEOXYRIBOSE SUGAR, ONE PHOSPHATE and ONE OF 4 NITROGENOUS BASES – GCAT 3. The deoxyribose sugar and phosphate form the sides of the ladder. The bases form the rungs. 4. The bases form specific pairs or SPECIFIC PURINE AND PYRIMIDINE COUPLES The COMPLEMENTARY BASE PAIRS are: Guanine (purine) and Cytosine (pyrimidine) Adenine (purine) and Thymine (pyrimidine) 5. The bases are held together by HYDROGEN BONDS. A and T form 2 hydrogen bonds while G and C form 3 hydrogen bonds. 6. The two strands run opposite to each other or are anti-parallel 7. The two strands are coiled in a regular manner forming a DOUBLE HELIX Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS DNA Replication DNA is the only molecule able to make a copy of itself. The process of duplication is called replication. Replication means to make a perfect identical copy. As we learnt in mitosis, when a cell divides the DNA must produce an exact copy of itself. This replication takes place during Interphase of mitosis. Mechanism of DNA replication 1. The double helix unwinds (or uncoils). 2. An enzyme (DNA helicase) unzips the two complimentary strands of DNA by breaking the bonds between the base pairs 3. Free complimentary nucleotides from the cytoplasm attach to the exposed bases on the separated strands. 4. A new complimentary strand is built along each ‘old’ strand – the old strand acts as a mould or template for the new DNA strand The new strands are assembled by the anabolic enzyme DNA polymerase. 5. Two new DNA strands are made. Each new DNA is a. half new DNA and half old DNA b. identical to the original DNA and to each other (Replication is semi-conservative) 6. Each new DNA rewinds to form a double helix Significance of DNA replication The fact that DNA is able to replicate or make an exact copy of itself means that the exact same DNA is passed on to each new generation of cells. Replication is the reason why both cells produced as a result of mitosis are genetically identical to each other and to the original parent. It is also the reason why every somatic cell of our body is genetically identical. Human beings grew from a zygote with 46 chromosomes by the process of mitosis and because of replication the new chromosomes passed on to the new cells are identical to that zygote and all the new cells produced are identical to each other and all contain 46 chromosomes. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS DNA profiling DNA profiling is a method of making a unique pattern of bands from the DNA of a person, which can then be used to distinguish that DNA from other DNA DNA profiling is also called genetic or DNA fingerprinting. Stages involved in DNA profiling 1. DNA isolation Cells are broken down to release DNA 2. DNA is cut into fragments The DNA is cut into fragments using special restriction enzymes e.g. one restriction enzyme always cuts the DNA at the base sequence GAATTC. The cut sections of DNA are called restriction fragments and are different lengths because the base sequences being cut may be close or far apart on the DNA strands 3. The DNA fragments are separated on the basis of size The fragments are separated by gel electrophoresis. An electric current drags the DNA fragments through a gel. The smaller fragments move further and faster through the gel and so the fragments are sorted by size into distinct bands in the gel. A photographic copy of the final pattern of DNA bands is obtained. 4. Patterns are compared The DNA profile is analysed and compared to another DNA profile to see if there are any similarities or differences. Two applications of DNA profiling 1. Medical DNA profiles can be used to solve paternity and maternity cases by comparing the child’s DNA profile with that of the disputed parent. Paternity cases are important in immigration, inheritance and rape cases All the childs bands match the father or the mother so the man is the father of the child The child’s bands only match the mother so the man is not the father. 2. Crime If biological material like hair, blood, semen or saliva is left at the scene of a crime, it can be used to prepare a DNA profile. This is then compared with the DNA profile of a suspect. If the patterns match then the suspect is associated with the crime. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS Genetic screening: screening Mistakes can happen in DNA replication. If a mistake happens in a gene, then the gene will not work properly and the resulting cell will not be able to make protein properly. If this happens in a somatic cell it might not be a big problem but if it happens in a gamete then every cell in the offspring will have the mutated gene and will be affected. This is how a genetic disease occurs. Genetic Screening is a diagnostic test to identify the presence or absence of changed genes Uses of genetic screening a. Adults screening - To see if a person is carrying an altered gene which may give rise to a genetic disorder To assess the risks of a genetic disorder being passed on to offspring b. Foetal screening – amniotic fluid around the baby is tested for the presence of genetic disorder Ethical issues a. If an embryo is tested and shown to have a genetic disorder it might encourage abortion b. Insurance companies and employers might discriminate against people who carry altered genes or have genetic disorders c. Should a person be told that they have a genetic disorder which would lead to death later in life? RNA (Ribonucleic Acid) RNA is a nucleic acid. It is mostly found in the cytoplasm in the form of ribosomes (rRNA). Messenger RNA (mRNA) is made in the nucleus using the code in DNA and it travels into the cytoplasm where it is involved in making protein. Structure 1. RNA is single stranded (half a ladder) 2. It has the sugar ribose 3. It has the bases guanine (G), cytosine (C), adenine (A) and uracil (U). It does not have thymine (T) 4. It is a polynucleotide (nucleotide polymer) RNA is a complementary structure to DNA because it is made using the code in DNA. If DNA has base sequence TAGCAT then the complementary RNA sequence will be AUCGUA. Differences between DNA and RNA DNA RNA Double stranded Single stranded Deoxyribose sugar Ribose sugar Thymine Uracil Self replicates Cannot self replicate Only in nucleus In cytoplasm, nucleolus and nucleus Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS 2.5.5 Protein Synthesis Protein synthesis happens as follows: • DNA contains the code for making protein It uses a sequence of three consecutive bases. Each group of three bases is called a triplet or codon. • This code is transcribed to mRNA Messenger RNA (mRNA) is made in the nucleus using the code in DNA. If DNA has the base sequence TAGCATGAG then the complementary mRNA will have the sequence AUCGUACUC. (The code is transcribed). • The transcribed code goes to a ribosome The mRNA leaves the DNA and moves out of the nucleus into the cytoplasm to a ribosome • The code is translated and the amino acids are assembled in the correct sequence to synthesise a protein As the mRNA passes through the ribosome each triplet of bases causes the correct sequence of amino acids to link together to make a protein. (The code is translated). • The protein folds into its functional shape. The protein is folded as it leaves the ribosome so it can carry out its function. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS H.2.5.15 PROTEIN SYNTHESIS (EXTENDED STUDY) 1. Transcription Enzymes unwind the DNA double helix at the site of a gene. The DNA code is transcribed on to a complementary mRNA strand. The nucleotides are assembled by the enzyme RNA polymerase Nucleus 2. The mRNA moves out of the nucleus and into the cytoplasm. It then moves into a ribosome. Codon 3. Translation Free-floating tRNAs with attached amino acids, in the cytoplasm, are attracted by their ‘binding site’ to the complementary mRNA attached to the ribosome. Amino acids are aligned in a sequence determined by the codons of the mRNA. The amino acids bond to form a new protein molecule The protein is released when the mRNA code sequence is complete Anticodon Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS Location of protein synthesis Protein synthesis takes place on the surface of ribosomes. Process of protein synthesis Protein synthesis involves 3 stages (a) Transcription (b) Translation (c) Protein Folding 1. Transcription Transcription is the making of mRNA using a DNA template Enzymes unwind the DNA double helix. RNA nucleotides bond with one strand of the exposed DNA. The enzyme RNA polymerase assembles these nucleotides to form mRNA. The mRNA, therefore, has a series of bases that are complementary to those in DNA. The mRNA moves into the cytoplasm. Each three base sequence of mRNA carries a genetic code or codon that specifies a starting codon, a particular amino acid or a stop codon. The mRNA attaches to ribosome subunits (rRNAs). These subunits form the ribosome which is the site of protein synthesis. 2. Translation Translation is the making of protein using the code in mRNA. Free-floating tRNAs with their attached amino acids, in the cytoplasm, are attracted by their ‘binding site’ or anti-codon to the complementary mRNA which is already attached to the ribosome. This ensures the amino acids are aligned in a sequence determined by the codons of the mRNA. The aligned amino acids bond to form the peptide links of the new protein molecule. The tRNAs continue to move to the ribosome, until a stop codon on the mRNA is reached. The protein is released when the mRNA code sequence is complete and the protein is synthesised. 3. Protein Folding Protein folding is the folding of protein into its functional shape. The protein is folded into its characteristic shape as it leaves the ribosome so it can carry out its function. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS 2.5.8 Evolution Definition of "evolution" Evolution is the way living things change genetically to produce new forms of life over long periods of time in response to changes in the environment Evolution is a gradual change in the characteristics of a species. It takes a long time and is related to changes in the environment. Eventually a new species is formed. Broad outline of Darwin and Wallace Theory of Natural Selection. Darwin suggested that evolution happens as follows: Observation 1 a. When organisms reproduce there are more offspring than parents. Observation 2 b. However, population numbers stay the same. Conclusion 1 c. This means that there must be competition for resources and a struggle to survive. Observation 3 d. The members of a population show genetic or inherited variations (differences). (These variations come from sexual reproduction and mutations). Conclusion 2 e. The organisms that have variations which allow them to adapt better to their environment will survive and reproduce. They will pass their genes to the next generation. Organisms with unfavourable variations will not survive and will not pass their genes on to the next generation. f. GRADUALLY the population changes and becomes BETTER ADAPTED to the ENVIRONMENT. g. Over a long time, so many changes happen that a NEW SPECIES is formed. This new species cannot breed with the original species. This theory of Natural Selection is called “survival of the fittest” and is basically about how well adapted organisms survive to reproduce and pass on their genes to the next generation. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS Evidence from any one source: fossil studies of any one anatomical characteristic Palaeontology is the study of fossils. Fossils are the dead remains of plants and animals that lived long ago and are preserved in rocks, peat or resin. Fossils are evidence for evolution because they can be placed in a series that show a gradual change over a long time and they can also show a change in response to a change in the environment. When the fossilised bones and teeth of horses are arranged in series we get a picture of the evolution of the horse. These fossils show changes in anatomical (body) characteristics like, increase in size, reduction in the number of toes, and the development of molar teeth (back teeth). These anatomical changes can also be related to changes in the environment The fossil records of horses are very well documented and really support the theory of evolution because 1. 2. 3. 4. They cover a long time span of 60 million years They shows changes in the height of the horse They can be related to changes in the environment They involve many species most of which are now extinct Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS 2.5.9 Genetic Engineering Genetic engineering is the artificial manipulation and alteration of genes The process of genetic engineering involves some or all of the following: Isolation cutting (restriction) transformation (ligation) introduction of base sequence changes expression. Isolation This involves removing the chromosome containing the target gene from a human cell and removing a plasmid from a bacterial cell. Cutting (Restriction) This involves cutting the chromosome and the plasmid with the same restriction enzyme. This means the target gene will easily fit into the plasmid as the cut ends will be complementary. Transformation (ligation) This is joining the exposed cut ends of the human DNA (target gene) and the plasmid DNA. They are joined using the enzyme DNA ligase. Introduction of base sequence changes This involves inserting the plasmid with the new DNA into a new bacterial host and replicating the host (cloning the cells). Expression This is the process where the bacterium with the new gene makes the required protein Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork 2.5 GENETICS Three applications of genetic engineering: one plant, one animal, one microorganism. Plant Application Some crops have had bacterial genes added to them. This makes the plants resistant to weedkiller (herbicides). This means that when weedkiller is sprayed on the crop the weeds will be killed but the crop is not affected Animal Application A human gene has been put into the DNA of sheep. This means that the sheep will produce a clotting chemical in their milk. This clotting factor is needed by haemophiliacs to clot their blood. Microoganism Application A human gene for making insulin has been inserted into the DNA of a bacterial cell. This means that the bacterial cell will make insulin. This is a protein needed by diabetics to regulate their sugar levels. Ms. B. Fennessy Loreto Secondary School Fermoy Co.Cork