This is a guest post from VCE student Katryna Induni for VCE Biology Unit 4 – Thanks Katryna! If you want to write a guest post on this blog, check out the information here.

DNA

• Deoxyribonucleic Acid
• Make up genes
• Double helix
• Subunits: nucleotides
• Each nucleotide has a sugar (deoxyribose), a phosphate part and a nitrogen containing base
• 4 different bases are Adenine, Thymine, Guanine & Cytosine
• DNA is built from nucleotides joined together to make a chain
• Chargaff’s rule: certain bases occur in equal proportions

Key Features of a double helix model of DNA

• Each DNA molecule consists of 2 nucleotide chains
• Chains run in opposite directions, said to be ‘anti-parallel’
• Weak hydrogen bonds form between base pairs
• The base pairs are said to be complementary; A-T & C-G

DNA Can:

• Act as a template strand for its own replication
• Contains genetic instructions
• Undergo change or mutation

Dissociation

• Heat DNA molecule to 90^oC
• Each molecule separates to form 2 single strands
• Heating doesn’t break the strong sugar-phosphate bonds that join nucleotides

Re-association

• If DNA is allowed to cool, complementary regions of the chains pair
• Hydrogen bonds reform
• DNA returns to double-stranded helix form
• Hybridisation – pairing between complementary DNA chains or parts of chains from different sources

Relating DNA to Chromosomes & Genes

DNA in Chromosomes

• Diploid human cells contain 23 pairs of chromosomes
• Each chromosome contains 1 double-stranded DNA molecule
• The longer the DNA the more genes it will carry
• Total amount of DNA in an average haploid cell is about 3000 million base pairs

DNA in a Gene

• Human cell has an estimated 20,000 to 25,000 genes
• Each gene consists of a length of double stranded DNA

Human Genome

• Total set of genes carried by an individual or cell

DNA in a Cell

• DNA is located in the nucleus
• Mitochondria also contain DNA
• Mitochondrial DNA is double-stranded circular molecule

Gene Structure

• A gene consists of part of a double strand of DNA
• Template Strand – 1 of the 2 chains contains the information present in a particular gene

Gene Sequencing

• Order of bases in a DNA fragment
• Different genes vary in the nucleotide sequences along their DNA
• Different genetic instructions within & between species are due to different nucleotide sequences in genes

Genetic Code

Information encoded in the genetic code

• Contains information for joining amino acids to form proteins
• 

• genes control the structure, biochemical and physiological functioning of an organism

Organisation of the Genetic Code

• Genes typically contain coded information for assembling amino acids to form proteins
• Proteins are made of combinations of 20 different amino acid sub-units
• One genetic instruction consists of a group of 3 bases, e.g. AAT,
• Because of this genetic code is called a triplet code

Main Features of the Genetic Code

• Pieces of information consist of triplets of 3 base sequences
• The code is non-overlapping
• Code is universal, same for all organisms
• Code is redundant, because more than one triplet of bases codes for one particular amino acid
• Information includes a start and stop instructions
• Start is TAC
• Stop is ATT, ATC, ACT

Genome

• Genome of an organism: complete set of genetic instructions, encoded in DNA
• Humans: DNA of the haploid set of autosomes and sex chromosomes
• Genomics is the study of genomes
• Prokaryotic DNA is single circular chromosome
• Genome sequenced mean the order of bases in the DNA has been identified

Human Genome Project

• Aim of sequencing the human genome and mapping all the human genes

Genetic Material

• Genetic material can change
• Mutation: any change in the arrangement or amount of DNA in a cell
• Mutations change the instructions encoded in genes
• Spontaneous mutation: when a causative agent cannot be identified
• Induced mutation: when a causative agent can be indentified
• Mutagenic agents include radiation such as X-Rays, ultraviolet radiation and nuclear radiation

Kinds of Mutations

• Substitution: replacement of one nucleotide by another in the DNA
• Addition: insertion of one or more nucleotides into the DNA strand
• Deletion: removal of one or more nucleotides from the DNA strand

Effects of a single base mutation

• Effects just one triplet in the genetic code
• May not have any effect because it still codes for the same amino acid- silent mutation
• Single base additions or deletions have major effect on genes involved because they affect not only that triplet code, but the ones after it- frameshift mutation

Trinucleotide repeat mutations

• Gene mutations involving the addition or deletion of a large number of bases
• Trinucleotide repeat expansion involves additional repeats of sequences, cause of several inherited conditions

Effect on next generation

• Somatic mutation: mutation which occurs in a body cell of an organism.
  only that cell & daughter cells will have mutation, not passed onto next generation
• Germline mutation: mutation occurring in a cell that produces gametes by meiosis
  inherited by next generation

DNA Repair

• Various DNA mechanisms involving one or more enzyme may reverse effects of a mutagenic agent

Closer Look at a Gene

• coding region of a gene: part of a gene containing coded information for making a protein
• flanking region: regions either side of coding region; before start is upstream, after is downstream

Introns: Just an interruption or two

• coding region is interrupted by other segments of DNA
• each segment of the coding region of a gene is called an exon
• exons are separated by lengths of DNA that don’t contain instructions
• non-coding segments are called introns

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I am Kat Induni, I finished Unit 3/4 Biology in 2009 and will complete my VCE in 2010. I am currently studying English, Chemistry, Further Maths, Latin and Legal Studies.  I hope to study Science/Arts at Monash and then study Medicine.