cellular control

• a mutation is a change in the genetic material which may affect the phenotype of the organism.
• a point mutation is a mutation affecting only one or very few nucleotides in a gene sequence.
• a substitution mutation is a mutation where one or more nucleotides are substituted for another in a DNA strand substitution mutation is
• a deletion mutation is a mutation where one or more nucleotides are deleted and lost from the DNA strand.
• an insertion mutation is a mutation where one or more extra nucleotides are inserted into a DNA strand
• frame shift: the deletion or insertion of a nucleotide or nucleotides leads to a frame-shift mutationIt shifts the reading frame of the sequence of bases as it will change every successive codon from the point of mutation
• a silent mutation is when the change of a single DNA nucleotide within a protein-coding portion of a gene does not affect the sequence of amino acids that make up the gene's protein.
• a nonsense mutation is the substitution of a single base pair that leads to the appearance of a stop codon where previously there was a codon specifying an amino acid.
• a missense mutation is a change in nucleotide sequence which changes the amino acid and alters the properties of the protein, two types: loss of function, gain of function
• gene mutations occur is single genes or sections of DNA
• chromosome mutations affect whole chromosomes or number of chromosomes in a cell
• The sequence of nucleotides could be mutated to a new sequence that codes for a different amino acid triplet and therefore a different protein meaning the new protein won't carry out the same functions
• Mutations can be silent, causing no change to proteins for example a mutation of a sequence of nucleotides could lead to no change in sequence of amino acids as the genetic code is degenerate.
• mutations can have a harmful effect when the phenotypoe of an organism is affected in a negative way as proteins are no longer synthesised or they are synthesised but are non-functional. For example, nonsense mutations.
• mutations can have a beneficial effect when a protein is synthesised that results in a new and useful characteristic. For example, a mutation in a protein present in the cell surface membranes of human cells means HIV cannot bind and enter.
• the three types of mutagen are:

  • physical (e.g. x-rays)
  • chemical (e.g. deaminating acids)
  • biological (e.g. viruses)
• the 4 types of chromosome mutation are: deletion, duplication, inversion, and translocation
• deletion- a section of chromosome breaks off and is lost within the cell
• duplication- sections get duplicated on a chromosome
• translocation- a section of one chromosome breaks off and joins another non-homologous chromosome
• inversion- a section of chromosome breaks off, is reversed, and then joins back onto the chromosome
• possible effects of a substitution inversion:

  No effect- no effect on the phenotype as normally functioning proteins are still synthesised 

  Damaging- the pehnotype is affected in a negative way because proteins are no longer synthesised or the proteins that are are non-functional. 

  Beneficial- the protein that is synthesised results in a new and useful characteristic in the phenotype
• possible effects if insertion and deletion mutations:

  Move or shifts the the reading frame of a sequence of nucleotides. This will change every successive codon from the point of mutation
• Gene expression- When the genetic information in DNA is converted into instructions for making proteins
• Epigenetics- the control of gene expression by the modification of DNA
• the 4 levels at which genes (or proteins) are regulated are: transcriptional, translational, post-transcriptional and post-translational
• Transcriptional- genes can be turned off or on eg. chromatin remodelling
• Post-transcriptional- mRNA can be modified which regulates translation and the types of proteins produced eg. RNA processing
• Translational- translation can be stopped or started eg. degradation of mRNA
• Post-translational- proteins can be modified after synthesis which changes their functions eg. folding or shortening of proteins
• chromatin- uncondensed DNA in a complex with histones
• heterochromatin- tightly wound DNA causing chromosomes to be visible during cell division
• Euchromatin- loosely wound DNA
• how chromatin remodelling allows the expression of some genes but not others:

  Transcription is not possible when DNA is tightly wound so genes in Euchromatin can be easily transcribed whereas genes in Heterochromatin cannot.
• Histones can be modified to increase or decrease the degree of packing. Therefore the DNA less tightly packed can be transcribed whereas the DNA tightly packed cannot be transcribed
• an operon is a group of genes that are under control of the same regulatory mechanism and are expressed at the same time
• Structural genes code for 3 enzymes and they are transcribed onto a single long molecule of mRNA. The enzymes are b-galactosidase, lactose permease and transacetylase
• The regulator gene codes for a repressor protein that prevents the transcription of the structural genes in the absence of lactose. The repressor protein binds to the operator and stops RNA polymerase binding to DNA and beginning transcription. RNA polymerase binds to the promoter region which releases the lactose/repressor protein complex from the operator region allowing transcription to begin
• how the lac operon works when lactose is absent:

  No lactose present to attach to the repressor protein so RNA polymerase cannot bind to the promoter region. Therefore no structural genes produced. This causes regulation.
• how the lac operon works when lactose is present:

  lactose binds to the repressor protein causing it to change shape so it can no longer bind to the operator. As a result RNA polymerase can bind to the promoter and start transcription of the structural genes.