Gene Expression

• DNA is the genetic code, a chromosome is a very long molecule of DNA.
• A gene is a section of DNA that codes for a particular trait or characteristic.
• An allele is an alternate form of a gene.
• Genotype is the combination of alleles for a particular trait, phenotype is the physical expression of the genotype.
• From the DNA, the cells make proteins which are expressed through traits.
• A triplet is a series of three bases in DNA, each triplet codes for a specific amino acid.
• Proteins are made in special organelles called ribosomes found in the cell’s cytoplasm.
• The ribosome is where translation takes place.
• Since the DNA is inside the nucleus the messenger RNA (mRNA) carries the “instructions” from the nucleus to the ribosome.
• RNA is a single-stranded molecule and has four nitrogen bases which are adenine, cytosine, guanine and uracil.
• The RNA carries the information of a single gene so the ribosome makes that particular protein when needed.
• The first step of protein synthesis is transcribing our DNA to the mRNA so information can safely leave the nucleus.
• The process of making a protein from mRNA is called translation.
• The base sequence code is translated into an amino acid sequence.
• When a cell needs a particular protein made, it sends a signal to a helicase enzyme which “unzips” the DNA at the start of a gene.
• The helicase breaks the hydrogen bonds between the base pairs, once the DNA is open and accessible, one of the strands is then read and used as a template.
• This strand is known as the template strand.
• Once the DNA is “unzipped” an enzyme adds nucleotides to the template strand, as the enzyme must follow the complementary base pairing.
• The new mRNA strand ends up with the same base sequence as the coding strand.
• The coding strand is the strand of DNA which is not used as a template to create mRNA.
• In mRNA, a series of three bases is known as a codon.
• Each codon eventually codes for an amino acid.
• Once mRNA synthesis is complete, the mRNA is broken off from the template strand by another enzyme and the DNA “zips” back up.
• Once the mRNA has been created, it leaves the nucleus through the nuclear pore (a special hole in the nuclear membrane).
• Both DNA and RNA are made up of sugar, phosphate and bases while proteins are made of amino acids.
• A protein is a long string of amino acids, each amino acid is coded for by the sequence of bases in DNA and subsequent codons in the mRNA.
• Every codon in the mRNA codes for one particular amino acid, the amino acid coded by a specific codon can be read off an amino acid table.
• Most amino acids can be coded through more than one codon, this is because of the redundancy due to degeneracy within the genetic code.
• Not all codons code for an amino acid, some codons tell the ribosome to “stop” (stop creating the protein) while others tell the ribosome to start.
• These codons are important so the ribosome creates proteins at the correct length.
• If the mutation occurs due to an environmental factor (e.g radiation, chemicals or viruses), it is called mutagens.
• Insertion and deletion mutation are opposites to each other, these mutations are more catastrophic as they result in every triplet ‘downstream’ (ocuring after the point of mutation) of the mutation being changed ( as each base is shuffled into a new codon)
• As most amino acids can be coded for by several different codons this means that substitution mutations do not necessarily result in a change to the amino acid sequence of the polypeptide.
• Mutation is a change in the base sequence of a gene.
• The majority of proteins are three-dimensional structures.
• The polypeptide chain is released and the mRNA-tRNA structure that was built to create the protein breaks down ready to form again when another protein is needed.
• This is the cause for several diseases and conditions such as albinism.
• This leads to evolution within the species in which the mutation occurs.
• The sequence of amino acids in the polypeptide chain changes.
• This process continues until the final product is made.