Biology (6.1): Cellular Control

mutation

mutation

random change to DNA base sequence

Causes of mutations (2)

1. random errors during DNA replication

2. exposure to mutagenic agents

Examples of mutagenic agents (3)

1. chemicals

2. ionising radiations

3. viruses

What are the two categories of mutations? (2)

1. substitution

2. insertion/deletion (indel)

What is a substitution mutation?

Where a base in a gene is changed for another

What are the three types of mutations (3)

1. silent

2. mis-sense

3. nonsense

silent mutation

the changed triplet codes for the same amino acid so the primary structure of the protein is not altered

What happens during transcription and translation if there is a change in DNA triplets?

mRNA codons change which could attract different tRNA anti-codons to the ribosome and produce a non-functional protein

Missense substitution mutation

new triplet codes for a different amino acid, so there’s a change to primary structure

Nonsense substitution mutation

new triplet codes for a stop codon so there is a significant change to primary structure

What are the three types of indel mutations? (3)

1. insertion

2. deletion

3. expanding triplet nucleotide repeat

Indel mutations

where base pairs are added or removed from the gene sequence

Deletion mutation

all triplets after the mutation are affected causing frame-shift and a significant change to the protein’s primary structure

Insertion mutation

all the triplets after the mutation are affected causing frame shift and a significant change to primary structure

Which mutations cause frame-shift?

insertion and deletion

Expanding triple nucleotide repeats

where whole triplets are added to the gene sequence changing the protein’s primary structure

What can mutations be? (3)

1. deleterious

2. neutral

3. beneficial

Examples of beneficial mutations (2)

1. lack of iris pigmentation in north-migrating early humans

2. decreased skin pigmentation in north-migrating early humans

What can deleterious mutations cause? (2)

1. genetic disorders (e.g. Huntington’s)

2. cancer

Examples of neutral mutations (3)

1. different shaped earlobes

2. facial dimple formation

3. variations in hair texture

Regulating gene expression

Control of the synthesis of a protein at the ribosome coded by genes

Why does gene expression need regulating?

1. genes only needed in certain conditions

2. some genes can be instructed to make more than one protein

3. some genes are active continuously

4. silencing of unrequired genes

5. gene products might be harmful in certain locations

6. genes only needed at a particular time

Example of genes that are only needed in certain conditions

genes for digestive enzymes

Example of genes that are active continuously

respiratory gene

Example of silencing unrequired genes

specialised cells using only certain genes

Example of genes that are needed only at a particular time

genes for development

When can regulation of gene expression happen? (3)

1. during transcription

2. post-transcriptional

3. post-translational

Gene regulation during transcription in prokaryotes

operons

Gene regulation during transcription in eukaryotes

transcription factors

Example of operons in prokaryotes

lac operon in E.coli

operon

a regulatory system in prokaryotes where a promotor controls the transcription of a cluster of genes

What are E.coli’s two food sources?

glucose and lactose

What proteins do E.coli need to survive in a non-lactose environment?

respiratory enzymes

What proteins do E.coli need to survive in a lactose-rich environment? (3)

1. respiratory enzymes

2. lactose transport proteins

3. lactase enzyme

What gene needs to be active in E.coli living in a non-lactose environment?

gene that codes for respiratory enzyme

What genes need to be inactive in E.coli living in a non-lactose environment? (2)

1. gene that codes for lactose transport proteins

2. gene that codes for lactase enzyme

What genes need to be active in E.coli living in a lactose-rich environment? (3)

1. gene that codes for respiratory enzymes

2. gene that codes for lactose transport proteins

3. gene that codes for lactase

How do operons switch genes off? (4)

1. regulator gene codes for a repressor protein

2. repressor protein binds to the operator

3. RNA polymerase cannot bind to the promotor

4. Repressor protein prevents RNA polymerase from transcribing genes

How do operons switch genes on? (4)

1. regulator gene codes for a repressor protein

2. an inducer binds to the repressor protein, changing its tertiary structure so it cannot bind to the operator

3. RNA polymerase can bind to the promotor

4. RNA polymerase moves along DNA and transcribes genes

regulator gene

codes for a repressor protein

repressor protein

binds to the operator

What does RNA polymerase bind to?

promotor

Parts of an operon (4)

1. regulator gene

2. promotor

3. operator

4. controlled genes

Lac operon

the regulatory system in E.coli where a promotor controls the transcription of two genes that allow the metabolism of lactose

Regulator gene in the lac operon

lacI

repressor protein in the lac operon

lacI protein

inducer in the lac operon

lactose

operator in the lac operon

lacO

controlled genes in the lac operon

lacZ and lacY

Transcription factors

proteins that bind to the promotor in eukaryotes, allowing RNA polymerase to bind and transcribe genes - activating them

How do transcription factors allow transcription of genes? (6)

1. hormone diffuses across cell surface membrane

2. hormone binds to inactive transcription factor

3. activating it

4. transcription factor binds with promotor

5. allows RNA polymerase to bind

6. mRNA transcribed for the gene

Gene regulation after transcription

alternative splicing

Exons

coding/expressed regions of DNA

introns

intervening regions of non-coding DNA

Splicing

the removal of introns from primary mRNA and joining of the exons to form mature mRNA in the nucleus

Alternative splicing

Splicing the exons of primary mRNA together in different orders to form many mature mRNA molecules

Where does alternative splicing happen?

nucleus

Gene regulation after translation

protein activation

Protein activation

the use of hormones and cAMP in a reaction called the second messenger system to activate proteins for use

Stages of the second messenger system (5)

1. hormone binds with receptor on cell surface membrane

2. adenylyl cyclase activates

3. catalyses conversion of ATP to cyclic AMP

4. activates PKA

5. adds phosphate groups to proteins to activate them using ATP

PKA

protein kinase A

homeobox genes

a family of regulator genes that code for transcription factors which activate genes responsible for development in eukaryotes

homeobox sequence

sequence of 180 base pairs found within genes that are involved in regulating patterns of anatomical development in animals, fungi and plants

What does the homeobox gene code for?

a transcription factor

What does the homeobox sequence code for?

the homeodomain

homeodomain

the DNA promotor binding site

highly conserved

a gene sequence that has remained unchanged in all descendent species throughout evolutionary history

Why are the sequences highly conserved?

1. ancient

2. controls multiple genes

What does it mean for the sequence to be ancient?

it is passed to many different species via an ancient common ancestor

What affect does the sequence controlling multiple genes have?

mutations cause significant change and are potentially fatal which prevents the inheritance of changes

Hox genes

homeobox genes found only in animals that control the body plan development

What do homeobox genes control to control development? (2)

1. cell cycle

2. apoptosis

Mitosis

Production of many identical daughter cells that can differentiate and form various tissues

Apoptosis

programmed death of cells using hydrolytic enzymes

Examples of how apoptosis removes excess cells formed during development (2)

1. digit separation

2. removing self-reactive B and T cells

Steps in apoptosis (5)

1. hydrolytic enzymes digest the cytoskeleton

2. chromatin condenses, nuclear membrane and DNA breaks apart

3. cell surface membrane begins to form blebs

4. cell breaks into vesicles

5. vesicles are ingested by phagocytes to prevent damage to other cells