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Gene Function - Genes in Action

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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