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.

Genes in Action

• Genes are made of DNA that contains information in coded form
• When genes are active, instructions are decoded and expressed in the phenotype
• Eukaryotic organisms, instructions are decoded in cytoplasm of cell
• Transcription- gene becomes active, makes a copy of the coded instructions it contains
• This copy leaves the nucleus and moves to the cytoplasm to be decoded (translation)

Transcription: Copying the Original

DNA & RNA Compared

Three Types of RNA

-          Messenger RNA (mRNA): carries genetic message to ribosomes, where the message is translated into a protein

-          Ribosomal RNA (rRNA): with proteins, makes ribosomes found in cytosol

-          Transfer RNA (tRNA): carry amino acids to ribosomes where they are used to construct proteins

DNA to RNA

• Transcription: synthesis of RNA from a DNA template
• Occurs in nucleus
• Pairing or hybridisation can occur between bases in one DNA Strand and complementary bases in an RNA strand
• DNA chain acts as a template to guide construction of RNA
• Process of transcription copies information of the DNA onto messenger RNA
• DNA is unwound by enzyme RNA Polymerase and exposes bases
• Template side of DNA is copied by RNA Polymerase
• Forms complementary codons (3 bases)
• Result: single stranded molecule of pre-messenger RNA
• This sequence is complementary to the base sequence of DNA template strand

Pre-mRNA is modified after transcription

• Pre-mRNA contains both introns & exons
• Regions of pre-mRNA that correspond to the introns in the coding region of the gene are cut out
• Produces a shorter mRNA molecule
• A methyl cap is added to one end of the molecule
• A poly-A tail is added to the other end to enable it to move to the cytoplasm to go to the ribosome

Translation: decoding genetic instructions

• Synthesis of a protein whose specific amino acid sequence is determined by the mRNA codons
• Ribosomes perform translation, IN the cytoplasm
• tRNA has an anti-codon located at one end of the molecule, and at the other end has a region which attaches to 1 specific amino acid
• tRNA contains anti-codons which match up to the mRNA codons, and they pair momentarily
• When the codon & anti-codon pair, the amino acid is deposited. A peptide bond forms between the amino acid & the growing polypeptide chain
• A stop codon will eventually be reached, where a protein molecule will occupy the codon site
• Bond between last amino acid & its tRNA molecule is broken, the ribosomal subunits break free from the mRNA molecule, releasing the completed protein

Alternative Splicing of pre-mRNA

• A gene can be regulated in different ways in order to produce more than 1 protein
• Introns retention can produce different mRNA molecules from the same pre-mRNA, depending on whether or not all the introns are cut out and discarded
• Exons, whether they are all used in the final mRNA
• Number of outputs from the genes is far greated than the number of genes in a genome

Comparing Prokaryotes and Eukaryotes

All Genes Produce RNA

Ribosomal RNA (rRNA)

• Produced in large quantities in the nucleus
• Stored in the nucleus forming nucleolus
• When rRNA is needed it leaves nucleus and goes to cytoplasm
• Genes located on short arms of human chromosomes produce rRNA

Genes have Various Functions

Structural & Regulator Genes

• Genes vary in function
• Structural genes: produce proteins which become part of the structure & functioning of the organism
• Regulator genes: produce proteins that control the action of other genes. Determine whether genes are active
• Genes which switch other genes on or off by producing proteins that act in one of 2 ways:
  1. DNA-binding proteins bind to regions of nuclear DNA near genes and directly switch genes on or off
  2. Proteins bind to receptors on membrane of cells in their target tissue and trigger a series of intercellular reactions that switch genes on or off, these are signalling proteins
• Homeotic genes: control embryonic development, produce DNA-binding proteins

DNA Replication

• DNA can self replicate
• Occurs before mitosis
• Occurs also in cells of gonads during early meiosis prior to gamete production

Process of DNA Replication

• Double stranded DNA unwinds to form a region of single stranded DNA- DNA Helicase
• Short length of RNA primers attach to the single DNA strands and begin replication
• Individual nucleotides are added to the 3’ end of the DNA strand according to base pairing rules- DNA Polymerase
• DNA leading strand forms continuously in the direction of replication
• DNA fragments formed on the lagging strand are joined by DNA Ligase
• Each of the new double helix molecules is an exact copy of the original DNA
• This process is semi conservative: each of the new double stranded molecules contains one of the original DNA strands and 1 new strand

Inactive & Active Genes

When are genes active?

• Of the 20,000 or so genes in a human cell, only some genes are expressed or ‘switched on’ at a given time
• Microarray: used to determine how active a cell is at a particular time. Thousands of single stranded DNA fragments, each corresponding to part of one particular gene, on a glass slide. Each strand acts as a probe for a particular fluorescent-tagged gene from a cell sample

Switching Genes Off

Silencing Genes via RNA Interference (RNAi)

• Adding double-stranded RNA to cells caused some genes to be silenced
• RNA interference doesn’t act directly on the DNA of genes
• Works by breaking down mRNA produced by one specific gene, not affecting any other genes
• Acts through short fragments of RNA
• Could create new classes of drugs to silence specific genes
• Treat inherited single-gene dominant disorders
• Could remove allergy causing proteins from food

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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.