biology module 6 cellular control

what is a gene mutation

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

how can the probability of a mutation occurring increase

ionising radiation such as X-rays can break the DNA strands

deaminating chemicals can alter the chemical structure of bases, converting one base into another

methyl or ethyl groups can be added to bases, leading to incorrect base pairing

why do most mutations not have an effect on us

most mutations do not alter the polypeptide or only alter is lightly so that its structure or function is not changed . many mutations happen in non coding sections of DNA and so have no effect on the amino acid sequence

what are the 3 main ways in which a mutation in the DNA base sequence can occur

1. insertion of one or more nucleotides

2. deletion of one or more nucleotides

3. substitution of one or more nucleotides

insertion

-new base randomly inserted into the DNA sequence

-changes the amino acid that would have bee coded for by the original base triplet

has a knock on effect known as a frame shift mutation

deletion

-nucleotide randomly deleted from the DNA sequence

-also has a knock on effect known as a frameshift mutation

substitution

-a base in the DNA sequence randomly swapped for a different base

-only changes the amino acid for the triplet in which the mutation occurs - does not cause a knock on effect

what are the three forms of substitution mutations

1. silent

2. missense

3. nonsense

silent mutations

doesn’t alter the amino acid sequence of the polypeptide because certain codons may code for the same amino acid

missense mutations

the mutation alters a single amino acid in the polypeptide chain

nonsense mutations

mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function. e.g cystic fibrosis is caused by a nonsense mutation

beneficial mutations

a small number of mutations result in a significantly altered polypeptide with a different shape . this may actually result in an altered characteristic in an organism that causes beneficial effects for the organism

harmful mutations

some mutations can lead to an altered characteristic in an organism that causes harmful effects for the organism

neutral mutations

offer no selective advantage or disadvantage to the organism

can occur either because :

-a mutation does not alter the polypeptide

-a mutation only alters the polypeptide slightly

what are the mechanisms used to make sure the correct genes are expressed in the correct cell known as

regulatory mechanisms

what are the three main types of regulatory mechanisms

1. regulation at a transcriptional level

2. regulation at a post transcriptional level

   1. regulation at a post translational level

what is a structural gene

codes for a protein that has a function within a cell

what is a regulatory gene

codes for proteins that control the expression of structural genes

what is an operon

a group/cluster of cells that are controlled by the same promoter

what is the lac operon

controls the production of the enzyme lactase. lactase breaks down lactose so it can be used as an energy source

what are the components of the lac operon

1. promoter

2. operator

3. lacZ

4. lacY

5. lacA

what is lacZ

structural gene that codes for lactase

what is lacY

structural gene that codes for permease to allow lactose into the cell

what is lacA

codes for transacetylase

what is located on the left of the lac operon

1. promoter for regulatory gene

2. lacI - codes for lac repressor protein

what are the two binding sites of the lac repressor protein and what happens when it binds to these places

1. the operator - prevents transcription of structural genes as RNA polymerase can no longer attach to the promoter

2. lactose - the shape of the repressor protein distorts and it can no longer bind to the operator

what happens when lactose is absent

• the regulatory gene is transcribed and translated to produce the lac repressor protein

• the lac repressor protein binds to the operator region upstream of lacZ

• due to the repressor protein, RNA polymerase is unable to bind to the promoter region

  • transcription of the genes does not take place and no lactase enzyme is synthesised

what happens when lactose is present

• uptake of lactose by the bacterium

• 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 the genes is translated and lactase is produced

what are transcription factors

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

how do transcription factors

• bind to the promoter region of a gene - this can either allow or prevent the transcription of the gene from taking place

• the presence of a transcription factor will either increase or decrease the rate of transcription of a gene

what are coding sequences of DNA

exons - will eventually be translated into the amino acids that will form the final polypeptides

what are non coding sequences of DNA

introns - not translated

what is splicing

• part of the post transcriptional modification

• as introns are not to be translated, they must be removed from the pre mRNA molecule

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

what does splicing ensure

only the coding regions of mRNA are used to form proteins by translation

what happens after polypeptides are formed by translation

they undergo modification in the Golgi apparatus or in the cytosol. some polypeptides may then require activation by cyclic AMP

what is cAMP

• cyclic AMP

• derived from ATP

• formed by the action of adenyl cyclase

what is an important role carried out by cAMP

• in eukaryotic cells, it activates protein kinase A

• PKA is an inactive precursor enzyme that can active other proteins when activated

what is the body plan of an organism

the basic pattern of the body

the polarity of an organism

segmentation of organisms into distinct body parts

when is body plan determined and what is it controlled by

• embryo stage of development

• controlled by a family of genes called homeobox genes

what do homeobox genes code for

transcription factors

how do transcription factors ensure the correct development of a body plan

• control which genes are being expressed at a particular time

• gene expression can be switched on or off in specific locations in a tightly controlled sequence during early development

  • cells in the correct locations differentiate into the correct cell types

what is a key feature of homeobox genes

they’re highly conserved - mutated alleles not passed on

what are hox genes

subset of homeobox genes that determine the identity of embryonic body regions along the head-tail axis

organised in groups called hox clusters

what is apoptosis and what happens during it

• programmed cell death

• enzymes digest cell contents

• cell breaks apart into small fragments

• phagocytes engulf any remains

how do mitosis and apoptosis control the development of an organism

• mitosis occurs in regions where more cells are needed

• apoptosis occurs in regions where cells need to be removed