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What is DNA? Chromosomes and their genes are made of a molecule called DNA. DNA stands for deoxyribosenucleic acid. Each chromosome is a very long molecule of tightly coiled DNA. DNA molecules carry the code that controls what your cells are made of and what they do. Structure of DNA DNA and base pairs Base pairs hold the two strands of the DNA helix together. The rules for base pairing are… A always pairs with T C always pairs with G There are millions of base pairs in a DNA molecule that always follow these rules. Amazingly, it is the sequence of bases along a DNA molecule that forms the genetic code - it’s that simple! What is mitosis? Mitosis begins with a single cell. How many chromosomes does this cell contain? original cell First the cell makes a copy of each chromosome… …then it divides. cell division Each new cell has a full set of chromosomes and is identical to the original cell. 2 new cells What is mitosis? Each new cell can keep on dividing by mitosis. Mitosis makes new cells for growth and repair in all living things. That’s how you get from one cell to 50 billion! Mitosis is also called copying division. What does this mean? Mitosis animation What is meiosis? Meiosis begins with a single cell. original cell First the cell makes a copy of each chromosome. Now, there are two stages of cell division… How many chromosomes do the new cells have compared to the original? first division second division 4 new cells What is meiosis? Meiosis produces four new sex cells with half the original number of chromosomes. In animals, meiosis produces ova in females and sperm cells in males. In plants the gametes produced by meiosis are pollen grains and ovules. Mitosis is also called reduction division. What does this mean? Meiosis animation How many chromosomes? Human sex cells have a set of 23 unpaired chromosomes - that’s half the usual number. Sex cells in animals and plants follow the same rules. They have half the usual number of chromosomes. They have a complete set of single chromosomes. Why is this important for fertilization and reproduction? Sex cells and cell division Sex cells have half the usual number of chromosomes. Why aren’t new sex cells produced by mitosis? Sex cells have to be produced by a special type of cell division. This is why there are two types of cell division. Sex cells are produced by meiosis. In which organs does meiosis take place? Inherited characteristics What makes this baby human? What determines its gender? In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents. So all human characteristics, including gender, must be something to do with chromosomes. Where are chromosomes found in cells? What are chromosomes? Chromosomes are long strands of genetic information located in the nuclei of cells. Chromosomes are most visible during cell division when they replicate and look like this… You will see chromosomes represented both ways. Homologous chromosomes In most cells chromosomes are matched in pairs based on their size and shape. chromosome from female parent chromosome from male parent homologous chromosomes Matching pairs of chromosomes are called homologous chromosomes. Where do homologous chromosomes come from? Each pair of homologous chromosomes contains one chromosome that has been inherited from each parent. Human chromosomes In human body cells there are a total of 46 chromosomes. How many pairs of homologous chromosomes are there in human body cells? 23 pairs of chromosomes You inherit half your chromosomes from your mother and half from your father. How many chromosomes do you inherit from each parent? 23 unpaired chromosomes What is meiosis? Meiosis begins with a single cell. First the cell makes a copy of each chromosome. Meiosis produces four new sex cells with half the original number of chromosomes. In animals, meiosis produces ova in females and sperm cells in males. In plants the gametes produced by meiosis are pollen grains and ovules. Meiosis is also called reduction division. What does this mean? Mitosis or meiosis? X and Y chromosomes There are two types of sex chromosome with names that are very easy to remember! X chromosome Y chromosome Females have two X chromosomes, one from each parent. XX Males have one X chromosome and one Y chromosome. Which parent does a baby boy inherit its Y chromosome from? XY Homologous chromosomes In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents. chromosome from female parent chromosome from male parent Chromosomes are matched in pairs that contain one chromosome inherited from each parent. So are the genes in a matching pair of chromosomes exactly the same? © Boardworks Ltd 2004 Different versions of genes The chromosomes in a matching pair contain the same type of genes that code for the same characteristics. gene for petal colour gene for petal colour version for red petals version for yellow petals Each chromosome may have a different version of a gene. Different versions of a gene, that code for different versions of a characteristic, are called alleles. Pairs of alleles – homozygous If the alleles in a matching pair are the same, they are called homozygous alleles. allele for yellow petals allele for yellow petals allele for red petals allele for red petals What colour are the flowers with these homozygous pairs of alleles? (Click twice on each bud to reveal the flower; click again to close them.) Pairs of alleles – heterozygous If the alleles in a matching pair are different, they are called heterozygous alleles. allele for red petals (Click twice on the bud to reveal the flower; click again to close it.) allele for yellow petals Which characteristic is expressed if alleles are different? Some alleles are dominant to other forms of a gene and will always be expressed. Which is the dominant allele in this heterozygous pair? Which is the recessive allele in this heterozygous pair? Representing alleles Letters are used to represent different alleles. A dominant allele is always a capital letter. allele for red petals = R A recessive allele is always the corresponding small letter. allele for yellow petals = r The allele pair for each characteristic is called the genotype. What colour are flowers with the genotype Rr? Genotypes and phenotypes The allele pair for each characteristic is called the genotype. The physical expression of an allele pair is the phenotype. What are the phenotypes of these genotypes? genotype: phenotype: (Click twice on each bud to reveal the flowers; click again to close them.) RR rr Rr Genetic jargon 1. Section of DNA that codes for a particular trait or characteristic. 2. A different form of a gene that codes for a different version of a characteristic. 3. A description of the pair of alleles present for a characteristic. 4. The physical expression of the alleles. Genetic jargon 5. Pair of alleles that produce a characteristic are the same, e.g. HH. 6. Pair of alleles that produce a characteristic are different, e.g. Hh. 7. An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. 8. An allele that will only be expressed when both alleles are of this type e.g. hh. Genetic jargon What do these genetic terms mean? gene 1. Section of DNA that codes for a particular trait or characteristic. allele 2. A different form of a gene that codes for a different version of a characteristic. genotype 3. A description of the pair of alleles present for a characteristic. phenotype 4. The physical expression of the alleles. Genetic jargon What do these genetic terms mean? homozygous 5. Pair of alleles that produce a characteristic are the same, e.g. HH. heterozygous 6. Pair of alleles that produce a characteristic are different, e.g. Hh. dominant 7. An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. recessive 8. An allele that will only be expressed when both alleles are of this type e.g. hh.