3- Genetics - GCSE Edexcel Biology

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of Thymine. The nucleotides link to form a strand of mRNA (messenger RNA) RNA polymerase AVAVA non-coding binding site XOVVVV AVA RNA polymerase binds to DNA and separates the strands. RNA polymerase moves along the DNA and starts making mRNA when it reaches the template strand. VVV non-template, or RNA polymerase UTABARU TGTCACH AATO direction of transcription newly made mRNA strand coding, strand of DNA nucleotides (containing a sugar called ribose) template strand of DNA Translation mRNA strands travel out of the nucleus through small holes in its membrane, calles nuclear pores. In the cytoplasm, the mRNA strands attch to ribosomes A ribosome moves along an mRNA strand three bases at a time. Each triplet of bases is called a codon. At each mRNA codon, a molecule of tRNA (transfer RNA) with complimentary bases lines up. Each molecule of tRNA carries a specific amino acid. As thr ribosome moves along, it joins the amino acids from the tRNA molecules together, forming a polypeptide. The polypeptide chain then folds up to form a protein with a specific shape tRNA free to collect another amino acid polypeptide chain forming amino acids CISIA AUS CAUCCULACCE tRNA brings next amino acid mRNA Ribosome moves along the mRNA in this direction, reading the code one codon at a time. 3.5- genetic variants and phenotypes Mutations A change in the bases of a gene creates a genetic variant or mutation> Environmental factors can also cause mutations. Mome mutations change an organism's phenotypes (physical charachteristics) Alleles are different versions of the same gene. mutations occur continuously some parts of DNA are non-coding so a mitation in these parts would not cause a change three types of mutations: • Substitution • Insertion • Deletion A change in DNA structure may result in a change in the protein synthesised by a gene A dew mutations code for an altered protein shape An enzyme may no longer fit the substrate binding site or a structural protein may lose it's strength 3.6- mendel most offspring look like a blend of their parents. Mendel observed that many charachteristics in pea plants, were either present or absent but not a 'blend' as people believed. He bred (crossed) pa plants together, observing charachteristics of the offspring He eventually concluded that inherited 'factors' control the variation of charachteristics. These 'factors' exist now as what we call alleles. Mendel's three laws of inheritance: each gamete recieves only one factor for a charachteristic the version of a factor that a gamete recieves is random and does not depend on the factors in the gamete • some versions of a factor are more powerful than others and always have an effect in the offspring 3.7- Alleles Alleles are different forms of the same gene. Since there are two copies of every chromosome in the body cell nucleus, there are two copies of every gene. There are many alleles for most genes and different combinations of alleles gives us different charachteristsics Homozygous-both alleles for one gene are the same Heterozygous- the alleles are different The alleles in an organism are its Genotype What the organism looks like is its Phenotype 3.8- Inheritance The two chromosomes that determine your sex are your sex chromosomes, there are two types of these, X and Y. A womans gametes al contain X chromosomes A man's gamestes contain both X and Y chromosomes Punnet squares are ways to show inheritance, allowing you to easily fine the ratio of outcomes, or the liklihood of an event happening as a probability female XX possible gametes X X XY male XX XY xx XY The boxes show the possible combinations in the offspring 3.9- Multiple and missing alleles Family pedigree charts chows how genotypes and their resulting phenotypes are inherited in families key Sex-Linked genetic disorders Disorders that show a different pattern of inheritance in men and women are called sex- linked genetic disorders e.g. colour blindness Qq Richard Mildred unaffected male ABO blood group system, classifies blood into one of four groups: A, B, AB, O mother's gametes Qq Diane XⓇ Robert Brian male with PKU X ? Sam father's gametes This is determined by certain 'marker molecules' that you have on the outside of your blood cells The gene that is responsible for the markers in the ABO system has three alleles and is written as I^A, I^B, I^o Both I^A and I^B are dominant. When both alleles for a gene affect the phenotype, we say they are codominant. xax Y (X*XXY Richard and Diane have two children X'Y Og Carolyn C in this Punnett square, the allele for colour blindness on the X chromosome is written as X' and the allele for normal colour vision is written as X". The Y chromosome does not have an allele for this gene. Emily unaffected female Carolyn has children with Brian (Richard and Diane's son) female with PKU 3.10- gene mutation A change in gene that creates a new allele is called a mutation. Mutations often occur during cell fivision Mutations happen when there is a mistake in copying DNA during cell division. Mutation can be advantageous, allowing some individuals to survive better in some envrionmental conditions The Human Genome project Objectives: • to map all of the genes in the genome • sequencing the base orer of each gene • increase speed of sequencing methods 3.11- variation Implications: improved diagnosis of disease • earlier detection of genetic predisposition to disease • rational drug design| • 'custom drugs' genetic variation is caused by the different alleles inherited during sexual reproduction Charachteristics can also be influenced by environmental variation. Charcahteristics that are changed by the environment during the life of the individual are called aquired charachteristics Variation can be grouped into two types: • discontinuous - data can only be taken from limited set of values • continuous - data can be any value in a range Continuous data for variation often forms a bell-shaped curve, known as a normal distribution. The number of times something occurs is its frequency. Variation in number of whole leaves is discontinuous because you can only have whole numbers, not values in between. When drawing a frequency bar chart for discontinuous variation, leave a gap between bars to make the discontinuity obvious. 8 7 6 2 Number of leaves on 2-month-old Begonia rex plants 10 11 12 13 14 15 16 Number of whole leaves 18- 16 14 12 10 Leaf length of Begonia rex plants 6 The chart is drawn with no gaps between bars, to show that the data are continuous Values are grouped to make it easier to see patterns in the data. Leaf length is a continuous variable as it can be any value 10 20 30 40 50 6.0 7.0 8.0 within the range. Leaf length (cm)-