Genetics and evolution

what is a gene mutation

a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

why do mutations occur

• Insertion of one or more nucleotides

• Deletion of one or more nucleotides

• Substitution of one or more nucleotides

what increases the probability of a mutation

• Ionising radiation such as X-rays

• Deaminating chemicals

• Methyl or ethyl groups can be added to bases

• Viruses can insert sections of viral DNA into the DNA of cells

types of substitution mutations

• Silent mutations – the mutation does not alter the amino acid sequence

• Missense mutations – the mutation alters a single amino acid

• Nonsense mutations – the mutation creates a premature stop codon

what are beneficial mutations

In some cases, this alteration to a polypeptide may actually result in an altered characteristic in an organism that causes beneficial effects for the organism

• e.g melanin production

what are chromosome mutations?

mutations affect whole chromosome

• deletion -section of chromosome breaks off, lost within the cell

• duplication - section of chromosome is duplicated

• translocation - section breaks off and joins another

• inversion - section breaks off and then joins back

what are the three levels of regulatory mechanisms

• Regulation at the transcriptional level (i.e. regulatory mechanisms that occur during transcription)

• Regulation at the post-transcriptional level (i.e. regulatory mechanisms that occur after transcription)

• Regulation at the post-translational level (i.e. regulatory mechanisms that occur after translation)

name the two types of genes

• A structural gene codes for a protein that has a function within a cell (e.g. enzymes, membrane carriers, hormones etc.)

  • For example, the F8 gene codes for the protein Factor VIII involved in blood clotting

• Regulatory genes code for proteins (or various forms of RNA) that control the expression of structural genes

describe the structure and function of the lac operon

the lac operon controls lactase production

contains:

• Promoter for structural genes

• Operator

• Located to the left (upstream):

  • Promoter for regulatory gene

  • Regulatory gene lacI that codes for the lac repressor protein

what happens to the lac operon when lactose is and isn’t absent?

absent:

• The regulatory gene is transcribed

• The lac repressor protein binds to the operator region

• Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region

• Transcription of the structural genes does not take place

• No lactase enzyme is synthesized

present:

• There is an uptake of lactose by the bacterium

• The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site

• RNA polymerase is then able to bind to the promoter region and transcription takes place

• The mRNA from all three structural genes is translated

• Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

regulatory mechanisms at the post transcriptional level

the editing of primary mRNA

• When transcription of a gene occurs, both the exons and introns are transcribed

• This means the messenger RNA (mRNA) molecule formed also contains exons and introns

• As the introns are not translated, they are removed

• The exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated

• This process is sometimes called ‘splicing’ and is part of the process of post-transcriptional modification

  

introns vs exons

• The coding sequences are called exons and

• The non-coding sequences are called introns

regulatory mechanisms at the post translational level

After translation, polypeptides undergo modifications in the Golgi apparatus

• addition of non- protein groups

• formation of disulphide bridges

• modification by cAMP - activates protein kinases. which catalyse the addition of phosphate groups to proteins

what are transcription factors?

bind to specific regions of DNA to control the transcription of genes

• bind to the promoter region of a gene, increase or decrease rate of transcription

what is a homeobox gene?

Homeobox genes are responsible for the genetic control of the development of body plans in different organisms

• they control the polarity of the organism

• segmentation of organisms such as insects and mammals into distinct body parts

DNA sequence that codes for a protein transcription factor

• The transcription factors attach to DNA at specific locations and regulate the transcription of genes

• Homeobox gene sequences are highly conserved

what are hox genes

• subset of homeobox genes

• determine the identity of embryonic body regions along the anterior-posterior axis (i.e. the head-tail axis)

• organised into Hox clusters

• Vertebrates have four Hox clusters (each containing 9-11 Hox genes), which are found on different chromosomes

• There is a linear order to the Hox genes is directly related to the order of the regions of the body that they affect

what is mitosis and apoptosis

• Mitosis is the type of cell division that produces identical new cells for processes such as growth, cell replacement and tissue repair

• Apoptosis is programmed cell death (sometimes referred to as natural cell death)

how does apoptosis work?

• The DNA of the cell becoming denser and more tightly packed

• The nuclear envelope of the cell's nucleus breaking down and chromatin condensing

• Vesicles forming that contain hydrolytic enzymes

• Phagocytes engulfing and digesting the cell via phagocytosis

why is mitosis and apoptosis important?

• key mechanisms controlling the development of body form

• some cells that are produced earlier on in development may no longer be needed

  • As a result, these cells are destroyed (by apoptosis) as part of the development of the organism

• For example, structures like fingers and toes first develop as a single combined unit and are then separated later via programmed cell death (apoptosis) of the cells in between the digits

what factors affect the expression of regulatory genes?

• change in temperature or light intensity

• psychological stress

• drugs