1.9 Genetic variation 2

Chromosomes are made up of DNA The chromosomes carry genes along them Genes code for proteins DNA is a triplet code; 3 bases = one amino acid Some bases are stop/start, otherwise nonsense (don't code for anything we know) Sequence of bases determines the protein produced - which gives the characteristic/trait Small differences in the DNA sequence of a gene = alleles result by mutation   DNA is a complex chemical that carries genetic information. DNA Is contained in chromosomes, which are found in the nucleus of most cells. The gene is the unit of inheritance and different forms of the same genes are called alleles. Chromosomes Chromosomes are x-shaped objects found in the nucleus of most cells. They consist of long strands of deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene.  The gene is the unit of inheritance, and each chromosome has several thousand genes.  We inherit particular chromosomes through the egg of our mother and sperm from our father.  The genes on those chromosomes carry the code that determines our physical characteristics, which are a combination of our two parents. The bases in the DNA molecule carry the different codes needed for different amino acids. The code for a particular amino acid is made from three bases in a particular order.   The backbone of the DNA molecule consists of two strands of alternating chemical strands of alternating sugar molecules and phosphate groups, the two strands twisted to form a double helix. Each sugar molecule is attached to one of the 4 bases called adenine, guanine, thymine and cytosine (A, G, T, C for short) The bases are paired up on opposite strands. A is always paired with T, and G is always paired up with C. This is called complementary base pairing - the order of bases in one strand determine the order of bases in another. A nucleotide is a basic building block for DNA made up of phosphate, sugar and base. The two strands are held together by weak hydrogen bonds. A gene consists of hundreds or thousands of bases. A gene codes for a particular protein by its particular base sequence.   DNA must copy or replicate itself before any cell division can take place. The DNA Double Helix is perfectly suited for replication because each strand can serve as a template to produce a strand opposite to itself. First it is "unzipped" - the two strands are separated. New nucleotides in the cell line up alongside the unpaired bases, A pairing with T and G pairing with C. The new nucleotides are joined together using an enzyme called DNA polymerase. DNA replication is semi conservative i.e. each strand in the original DNA molecule is used as a template o make a new strand of DNA. Each new DNA molecule contains an original strand and a newly made strand.

Crossing over - bits of homologous chromosomes are exchanged/swapped during meiosis. Independent assortment - it is random which one of a pair of homologous chromosomes goes into a gamete after they line up in pairs during meiosis.   Each parent cell has a pair of homologous chromosomes, one homolog from the father, and one from the mother. In meiosis the maternal and paternal chromosomes can be shuffled into the daughter cells in many different combinations. This ensures genetic variation in sexually reproducing organisms. Further genetic variation comes from crossing over during prophase I. In prophase I of meiosis, the replicated homologous pair of chromosomes comes together in the process called synapsis, and sections of the chromosomes are exchanged. You can see that after crossing over, the resultant chromosomes are neither maternal or paternal, but contain genes from both parents.   When homologous pairs of chromosomes line up during meiosis, they do so randomly. This means it is completely random which combination of alleles end up in a particular gamete. This process is called independent assortment.

Permanent change in DNA sequence or number of chromosomes Difference in base sequence -> Different amino acid -> different protein Change in order = slightly altered gene Alternate for of same gene is called an allele Mutations can only be passed on if they are in a gamete / sex cell   Changes to genes are called mutations. Mutations can be spontaneous (they just happen). They can also happen because of: Radiation Chemicals such as tar from cigarette smoke   Mutations may have no effect. For example, the protein that a mutated gene produces may work just as well as the protein from the non mutated gene. Mutations may sometimes be helpful but they re often harmful. For example, haemophilia is an inherited disorder that stops blood from clotting properly. It is caused by a mutated gene. hh Genes can be switched on and off. In any one cell, only some of the full set of available genes are used. Different types of cells produce different ranges of proteins. This affects the functions they can carry out. For example, only pancreas cells switch on the gene for making insulin.

The big picture

Crossing over and independant assortment

Mutation