AS Demonstrate understanding of genetic variation

 Make sure that you look at the explanatory notes in your workbook and the specific learning outcomes as this will allow you to focus on what you need to be learning.

 DNA – its inheritance, continuity of life.  DNA and variation  Variation and evolution  DNA determining characteristics  DNA, genes, alleles and chromosomes  Genotypes and phenotypes  Homologous chromosomes and inheritance of two copies of a gene  Alleles as versions of genes

 Role of mutations in forming new alleles  Meiosis and the production of gametes  Sexual reproduction and the production of variation.  Monohybrid crosses showing complete dominance, sex determination and genotypes and phenotypes ratios  Inheritable and non-inheritable characteristics  Differing rates of survival depending on phenotype  Variation in populations and survival  Advantages and disadvantages of sexual reproduction

 Gene  Allele  Mutation  Genotype  Phenotype  Gamete  Zygote  Dominant  Recessive  Homologous  Heterologous  Pure breeding  Punnett square  Pedigree chart

 DNA is Deoxyribonucleic acid  If is found in the nucleus of living cells  It is the chemical substance that codes for the inheritable characteristics of living things, like eye colour.  DNA is a double helix; like a twisted ladder so that it takes up less room; it is very long.  The DNA molecule is made up of nucleotides joined together by chemical bonds.

 Deoxyribonucleic acid.  Actually two molecules held together by hydrogen bonds between base pairs.  A double helix.

 There are three parts to a nucleotide: 1. A ribose sugar unit 2. A phosphate group 3. A base unit, there are 4 types.

 The backbone of the DNA molecule is made out of the sugars and phosphate groups.  The rungs are made out of the bases paired up by hydrogen bonds.

 There are four bases that make up the rungs:  A – adenine  T – thymineA-T pair  C – cytosine  G – guanine C-G pair  The bases always bond this way  Write the complementary sequence for this strand: AAC GCT ATA CGA TTA GCG TCG

THE BASE PAIRS  Notice how there are two hydrogen bonds between A and T and three between G and C.  This is why A can’t bond with C etc.

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 A molecule of DNA is very long. Human DNA is about 5 billion bases long. It is the order of the bases that codes for the characteristics of an organism.  The DNA is organised into triplets. A triplet is a series of three bases, such as ATC. Each triplet codes for a amino acid. An amino acid is a building block of proteins. So, The order of triplets on a piece of DNA will specify the order of amino acids, and therefore the protein being made.

 DNA is passed from one generation to the next and provides all the information for the cells in a living organism to develop and carry out their functions.  The genetic code for organisms is given by the sequence of bases (specifically triplets) on the DNA strand.  The characteristics of organisms is determined by the genetic code.

 Traits or characteristics are determined by Genes.  A gene is a length of DNA made up of thousands of base pairs that codes for one characteristic.  Unique code  unique gene  unique characteristic.  One gene = one characteristic  A gene codes for the production of a protein.  The protein contributes to the characteristic, eg, it may be an insulin protein to help glucose get into cell, or a pigment that colours your hair.

 Thread-like structures bearing genes that are found in the nuclei of cells.  Visible with a microscope during prophase of cell division when the chromosomes are seperating.  Humans have 46 chromosomes, 23 pairs.  Chromosomes are made of DNA (Deoxyribonucleic acid).  Chromatin is how the DNA is found in the cell when it is not dividing.

 22 autosomal (non sex chromosomes) pairs, one of each pair is from each parent.  Chromosomes are numbered, 1 – 23.  One pair of sex chromosomes: male has X from mum and Y from dad, girl has an X from both parents.  If the cells that make sperm and egg do not divide properly then an organism may have missing chromosomes or extra chromosomes or broken chromosomes.

 New cells are made all the time for growth and repair of tissues and every new cell needs the same DNA as the original.  DNA needs to be copied before a cell can divide into two – DNA replication

 The double helix unwinds and the two strands of DNA separate  The two original strands of DNA act as templates to make new matching strands.  New nucleotides from within the nucleus, line up according to their base pairing with the original strand, A with T and G with C.  The nucleotides are joined by enzymes according to the base pairing rules.  Each new piece of DNA (or chromosome) is made of an old and a new piece of DNA (semi- conservative).  There are enzymes that check that replication is correct to avoid mutations occurring. 

 A supply of raw materials – free nucleotides  Energy from cellular respiration  Enzymes to catalyse the process  Information in existing DNA

 Mitosis is the making of normal body cells and occurs in every living thing and every part of an organism, eg, check cells.  Cell division for growth and repair  Two identical daughter cells are produced from one cell with identical DNA to the parent cell.  After mitosis all cells will have same number of chromosomes as the parent cell and each other.  y/mitosis.html y/mitosis.html

 Chromosomes shorten and thicken and become visible and they replicate  Centromeres of chromosomes line up along equator.  Spindle fibres attach to the chromatids at their centromeres and pull the chromosomes towards the poles.  Two identical groups of daughter chromosomes at each end of cell.  the cell membrane grows inwards and cytoplasm divides to make two identical daughter cells

 Once the DNA is replicated the chromatids will separate during cell division.  Ensures that the daughter cell produced has exactly the same genetic material.  If it didn’t then things can go wrong, eg, cancer.  A tour of DNA A tour of DNA

 A gene is a sequence of DNA bases located at a specific place on a particular chromosome, called its locus  All the genes an organism has is called its GENOME.  They direct how an organism is made and maintained.  Each gene codes for the building of one protein.  Genes for a particular characteristic are found in the same position on homologous chromosomes (a pair).  Alleles are different versions of a gene: eg, eyecolour – you may have the blue allele, brown allele or maybe both!  Different alleles have slightly different sequences of bases, resulting in slightly different proteins being made.

 Proteins are really important because everything in a cell is either a protein, or made by enzymes which are proteins.  Proteins are made out of amino acids joined together.  There are only 20 types of amino acids. The order of amino acids changes the protein.  Some important proteins are: collagen, keratin, haemoglobin, insulin, melanin, oestrogen, testosterone.

 Meiosis is cell division to produce sex cells – sperm and eggs.  The daughter cells are genetically different from each other.  The daughter cells have only half the number of chromosomes as the parent cell  There are two divisions, whereas mitosis has one.  Occurs in mature organisms – in mammals it occurs in the ovaries and testes  Occurs in the stamens and ovules in plants

1. Chromosomes shorten and become visible 2. They come together in homologous pairs and crossing over occurs 3. Segregation - A spindle forms and homologous chromosomes (each consisting of two chromatids) are pulled apart to opposite poles. 4. Each cell splits into two daughter cells. 5. Each daughter cell has the haploid number of chromosomes, but each chromosome is made of two chromatids. 6. A new spindle forms and chromosomes are arranged on the equator 7. Chromatids separate to the cell ends 8. Each daughter cell splits into two to finally form four daughter cells, each with half the number of chromosomes.

 Half the genetic material in an offspring comes from the sperm and half from the egg.  The joining of a sperm and egg is random producing variation  Crossing over during meiosis produces variation as the original chromosomes now have different alleles as portions of the homologous chromosomes have swapped.  The offspring will have characteristics unique to itself depending on the combinations of alleles for each of its features.

 Sex is determined by two sex chromosomes carried by the sperm and the egg and is determined at fertilisation.  Y male, X female  Males – XY females have XX  The X and Y chromosomes act as homologous chromosomes during cell division and segregate like normal during the first division of meiosis.  Each gamete (sperm or egg) gets only one sex chromosome.  Half a man’s sperm have X and half have Y  All egg contain X chromosomes.

 Characteristics – features a living thing has, eg, black fur. = traits.  Can be inherited – eg, gained from parents by transmission of genes.  Can be acquired – eg, permed hair.  Characteristics Genetic variation within a species enables the species to survive changes in habitat better. There will always be a few organisms that can survive, eg, antibiotic resistance in bacteria.

 Variation, the small differences that exist between individuals, can be described as being either CONTINUOUS OR DISCONTINUOUS.

 I ndividuals 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 groups are a good example: you are either one blood group or another - you can't be in between.  controlled by alleles of a single gene or a small number of genes. The environment has little effect on this type of variation.

 Evolution is a change of representation of specific versions of genes in a population which leads to changes in the characteristics of organisms.

 There is a complete range of measurements from one extreme to the other.  Height is an example of continuous variation  Other examples of continuous variation include: Weight; Hand span; Shoe size; Milk yield in cows.  Combined effect of many genes (known as polygenic inheritance).  Often significantly affected by environmental influences.