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Nature, Structure & Organisation of the Genetic Material

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 90oC
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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