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GENETICS DR. PETER VICKERS 1 WHAT ARE GENES? GENES ARE SECTIONS OF DNA CARRIED ON OUR CHROMOSOMES WHICH CONTAIN PARTICULAR SETS OF INSTRUCTIONS RELATED TO OUR GROWTH, DEVELOPMENT, REPRODUCTION, FUNCTIONING AND AGING 2 SOME DEFINITIONS DNA = Deoxyribonucleic Acid – makes up the double helix RNA = Ribonucleic Acid – along with mRNA (below) determines the amino acid composition of proteins, which, in turn determines the function of the protein, and therefore the function of that particular cell mRNA = messenger Ribonucleic Acid Chromosome – see later slide 3 CHROMOSOME • A chromosome is a complicated strand of DNA and protein • Each chromosome is made up of 2 chromatids joined by a centromere • Each cell in our body which has a nucleus contains, within its genes, all the genetic information to make an entire human being 4 NUCLEOTIDES • Consist of 3 molecules: • 1. Deoxyribose - a 5 carbon cyclic sugar • 2. Phosphate - inorganic negatively-charged molecule • 3. Base - a nitrogen-carbon ring structure 5 BASES • There are 4 different bases found in DNA • adenine (A) • thymine (T) • guanine (G) • cytosine (C) 6 7 BASES & DOUBLE HELIX • The order of the bases along the length of the DNA molecule provides the variation that allows for information storage • The golden rule: • adenine always pairs with thymine • guanine always pairs with cytosine 8 QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. 9 CHROMOSOMES • The nuclear DNA of eukaryotes is associated with protein • • • • 10 molecules called HISTONES Together, these 2 components make up the NUCLEOSOMES contained within the cell nucleus. This nucleic acid-histone complex is known as CHROMATIN Chromatin is tightly folded because the DNA molecules are very long would not fit in the cell otherwise Unravelled it would stretch to the moon and back about 8,000 times 11 CHROMOSOMES • Each chromosome is made up of 2 chromatids • • • • 12 joined by a centromere In most humans, each nucleated body cell has 46 chromosomes, arranged in 23 pairs One pair determines the sex of a person Females have a matched homologous pair of X chromosomes Males have an unmatched heterologous pair - one X and one Y chromosome 13 • The remaining 22 pairs of chromosomes are known • • • • • 14 as AUTOSOMES (which determine physical/body characteristics and others) These are homologous since both members of the pair are usually identical in shape and size The position a gene occupies on a chromosome is called a LOCUS There are different loci for colour, height, etc. Genes that occupy corresponding loci are called ALLELES An allele determines an alternative form of the same characteristic GENES • A person with a pair of identical alleles for a particular gene locus is HOMOZYGOUS for that gene locus • One with a dissimilar pair is HETEROZYGOUS • A DOMINANT GENE is one which exerts its effects when it is present on only one of the chromosomes • A RECESSIVE GENE has to be present on both chromosomes to manifest itself physically 15 NUCLEIC ACIDS • Nucleic acids have 2 major functions: • 1. The direction of all protein synthesis • 2. The accurate transmission of this information from one generation to the next, and from one cell to its daughter cells 16 PROTEIN SYNTHESIS • Cells make proteins by translating the genetic information encoded in DNA • The genetic information in a region of DNA is copied to produce a specific molecule of RNA • Through a complex series of reactions, the information contained in RNA is translated into a corresponding specific sequence of amino acids in a newly produced protein molecule 17 PROTEIN SYNTHESIS • There are, therefore, 2 principal steps in protein synthesis: • 1. Transcription • 2. Translation 18 MEERA NAAM PETER HAI QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. MY NAME IS PETER 19 TRANSCRIPTION • The process by which genetic information encoded in DNA is copied by a strand of mRNA • By using a specific portion of the cell’s DNA as a template, the genetic information stored in the sequence of nitrogenous bases of DNA is rewritten so that the same information appears in the nitrogenous bases of mRNA 20 • CYTOSINE in template = GUANINE in mRNA • GUANINE in template = CYTOSINE in mRNA • THYMINE in template = ADENINE in mRNA • ADENINE in template = URACIL (U) in mRNA • E.g. if DNA has base sequence AGGCAGTGC • Then mRNA has base sequence UCCGUCACG 21 key guanine cytosine adenine thymine QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. 22 QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. 23 TRANSLATION • The process by which information in the nitrogenous base of mRNA is used to specify the amino acid sequence of a protein. 24 QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. 25 SUMMARY • As each ribosome moves along the mRNA strand, it ‘reads’ the information coded in the mRNA and synthesises a protein according to that information • The ribosome synthesises the protein by translating the codon sequence into an amino acid sequence • Protein synthesis progresses at the rate of about 15 amino acids per second 26 • As the ribosome moves along the mRNA strand, and before it completes translation of that gene, another ribosome may attach itself and begin translation of the same mRNA strand • Several ribosomes moving simultaneously in tandem along the same mRNA molecule permit the translation of a single mRNA strand into several identical proteins simultaneously • Thus we can define a GENE as a group of nucleotides on a DNA molecule that serves as the master mould for manufacturing a specific protein 27 • Genes average about 1,000 pairs of nucleotides which appear in a specific sequence on the DNA molecule • No two genes have exactly the same sequence of nucleotides, and this is the key to heredity • c) each gene is responsible for making a particular protein, as follows: transcription • • 28 DNA translation RNA protein DNA double helix DNA coding strand QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. transcription Messenger RNA translation protein The flow of information from DNA to RNA to protein 29 CELL DIVISION • In order for the body to grow and to replace cells which have died, cells must be able to reproduce themselves • In some organisms, this can occur by simple fission • The nucleus in a single cell becomes elongated and then divides to form 2 nuclei in one cell • The cytoplasm then divides in between the 2 nuclei to form 2 daughter cells, each with its own nucleus 30 FUSION OF THE GAMETES • When the gametes, each with 23 chromosomes, fuse together, a cell known as a ZYGOTE with 23 paired chromosomes (i.e. 46 in all) is formed • One chromosome from each pair comes from the mother, and one from the father • The zygote cell then divides by mitosis many times to form the embryo 31 MENDELIAN GENETICS • 1865 - Gregor Mendel - a monk living in Brno • Wanted to produce the finest peas • Studied them extensively • Introduced 3 novel (for that time) approaches to study: – Not only observed, but experimented – Then he counted (statistics) – Ensured original parental stocks were pure breeding stocks 32 • Mendel demonstrated that members of a pair of ALLELES separate clearly during meiosis • We all possess a pair of genes (alleles) at any one LOCUS, but we can only pass on ONE of these genes to our child FATHER Aa • AB 33 Ab MOTHER Bb aB ab Another way of representing genetic inheritance is by means of a PUNNET SQUARE Aa AB aB Bb Ab 34 ab • The previous diagram explains Mendel’s FIRST LAW • This is that the father can pass either gene A or gene a, and the mother gene B or gene b • But each child can only inherit either gene A or gene a and gene B or gene b • Therefore statistically, there is a 1 in 4 (1:4) chance that any child of these two parents will have genes AB, Ab,aB or ab 35 MENDEL’S SECOND LAW • Members of different pairs of alleles sort independently of each other during gametogenesis • Each member of a pair of alleles may occur randomly with either member of another pair of alleles 36 DOMINANCE • At each locus, the 2 alleles can be either DOMINANT or RECESSIVE • Dominant genes are always expressed by the next generation if they are passed on • Recessive genes are only expressed if they share a locus with another recessive gene • Note: in genetic representations, dominant genes are always given capital letters, whilst recessive genes are given lower case letters 37 • Look at our Punnet Square again Aa AB aB Bb Ab 38 ab •75% of the offspring carry at least one dominant gene, and 25% carry only recessive genes. •Therefore, 75% will express a dominant gene, and 25% a recessive gene. AUTOSOMAL DOMINANT INHERITANCE • If a dominant gene is one that causes a medical disorder, for example Huntington’s Disease or Neurofibromatosis, then there will be a 50% risk of having that medical condition • There will thus be a 50% of not having the condition • This mode of inheritance is known as Autosomal Dominant Inheritance 39 AUTOSOMAL DOMINANT DISORDERS • Apart from the odd disorder, such as Huntington’s Disease, Autosomal Dominant disorders are generally not as fatal or as severe as Autosomal Recessive disorders • Why Not? 40 AUTOSOMAL RECESSIVE INHERITANCE Aa AB aB Ba Aa aa Returning to the Punnet Square - if the 2 recessive genes (aa) are coded for the same medical disorder, then the chances at each pregnancy of having an affected child are: 41 • For an affected child = 25% (1:4) • For a carrier = 50% (1:2) • For a child who is neither affected nor a carrier = • • • • • • • 42 25% (1:4) ____________________________________ Remember that these odds occur for each pregnancy, so you could have: 4 children and have 1 affected, 2 carriers, and 1 unaffected Or 4 carriers Or 3 affected and 1 carrier And so on ... The odds are the same for each child! AUTOSOMAL RECESSIVE DISORDERS • Examples of Autosomal Recessive disorders are: • Cystic Fibrosis • Thalassaemia • Severe Combined Immunodeficiency (SCIDS) • Spinal Muscular Atrophy 43 X-LINKED DISORDERS • As well as autosomal inheritance, we can inherit disorders via the sex chromosomes • For a girl, these are XX • For a boy they are XY • The main role of these chromosomes is to determine the sex of a baby 44 MOTHER XX FATHER XY • • XX XY XX XY • What are the chances of a couple having a boy or a girl? • The chances are 50% for a girl and 50% for a boy 45 • Suppose the mother carried an abnormal, recessive gene on one of her X chromosomes – here denoted by the green ‘X’ • MOTHER XX • XX 46 FATHER XY XY XX XY • The Y chromosome is unable to ‘block’ any of the genes on the mother’s chromosome • Therefore the possibilities in any one child are: • 1. One girl would not be a carrier or be affected • 2. One girl would be a carrier, but not affected • 3. One boy would not be a carrier or affected • 4. One boy would be affected 47 • Thus we can say, with regard to X-linked disorders: • 1. Only boys are affected, and there is a 50% • • • • • • 48 chance that they will be affected, but 100% chance that they will not be carriers 2. No girls are affected, but there is a 50% chance that they will be a carrier Examples are: Haemophilia Duchenne Muscular Dystrophy X-linked Agammaglobulinaemia Red-Green Colour Blindness HLA TYPING - TT • * Parents are 1st. Cousins • Father A9,24 • A11 • Mother A29 • A9,24 • TT A11 • A2 • FT (sister) A9,24 • A2 • RT (sister) A11 • A2 49 B40 B35 B40 B40 B35 B40 B40 B40 B35 B40 DR2 DR2 DR2 DR2 DR2 DR2 DR2 DR2 DR2 DR2 HLA TYPING - GO • * Parents say they are not related • Father A1 A11 • • Mother • GO • 50 A1 A 24 A11 B8 B35 B8 B13 B35 A1 DR3 DR7 DR3 DR7 DR7 B8 DR3 51 chromosomes. (n.d.). The American Heritage® Science Dictionary. Retrieved August 26, 2014, from Dictionary.com website: http://dictionary.reference.com/browse/chromosomes 52