Cellular control

Mutations= Change in the DNA base sequence caused by mutagens.

Mutagens = UV, X-rays, gamma rays, radiation, chemicals.

Point mutations:

  • Substitution,

  • Addition,

  • Deletion.

They only affect one base.

Addition and deletion cause a frameshift mutation which disrupts the triplet code reading.

Effects of mutations:

A mutation can be neutral or silent, it has no effect due to the degenerate nature of the codon.

A mutation can be beneficial which can enhance the function of the protein, however, this is very rare.

A mutation can be damaging: cancer, malfunctioning protein made.

More mutations:

Nonsense mutation= codon has changed into a stop codon.

Missense mutation= amino acid A turned into amino acid B, degenerate coding has failed.

Chromosomal mutations:

Deletion- deleting part of a chromosome.

Duplication- having part of a chromosome doubled.

Translocation- a section moved itself from one chromosome to another.

These mutations can all affect the protein that is made and its function.

Transcription factors are proteins involved in the process of transcribing DNA into RNA.

Transcriptional control of gene expression:

Turn genes on and off by altering conditions to allow RNA polymerase to bind in eukaryotes.

Heterochromatin= DNA is tightly wound around histone proteins to make it compact. RNA polymerase cannot bind, no transcription occurs. Changed into euchromatin, DNA is loosely wounded, gene is accessible for RNA polymerase to bind, transcription can occur. Can change into euchromatin through phosphorylation.

Lac operon in prokaryotes:

Operon= A group of genes controlled by the same regulatory mechanism and expressed at the same time.

Structural genes (Lac Z, Y, A) - Proteins not involved in DNA regulation:

  • B-galactosidase,

  • Lactose permease,

  • Transacetylase.

These enzymes metabolise lactose.

Regulatory genes (Lac I)- Proteins involved in DNA regulation:

  • Repressor protein will prevent transcription of these structural genes to make those enzymes.

The operator region is the DNA sequence where the repressor protein binds.

The promoter region is where the RNA polymerase will bind.

The binding of these proteins will affect if transcription occurs or not.

With glucose:

  1. When glucose is present, Lac I is expressed to make the repressor protein. The repressor protein binds to the operator region.

  2. Due to its shape and size, it blocks the RNA polymerase from binding to the promoter region. Hence, transcription of the structural genes cannot occur.

  3. If glucose is not present, lactose binds to the repressor protein, causing a conformational change, the repressor protein is released from the operator region.

  4. RNA polymerase can bind to the promoter region as it is unblocked, it can now transcribe the three structural genes. Transcription can occur.

  5. We can make this process occur more efficiently by:

    Receptor protein ( CRP ) binds with cyclic AMP and upregulates the activity of RNA polymerase to make transcription more efficient.

  6. The upregulated RNA polymerase transcribes the structural genes to make B-galactosidase, lactose permease and lactose transacetylase.

Prokaryotes prefer glucose over lactose. The presence of glucose causes lactose to be released from the repressor protein causing a conformational change. The repressor protein binds to the operator region once more, hence the RNA polymerase cannot bind to the promotor region to start transcription.

Glucose also leads to a decrease in cAMP concentration in the cell, therefore less CRP-cAMP complexes are made available to bind to RNA polymerase, this downregulates its transcription action.

These enzymes metabolise lactose. Lactose permease facilitates the passage of lactose across the plasma membrane. Breaks down lactose into galactose and glucose.

Post transcriptional control of gene expression- Modification of mRNA:

  1. Splicing occurs to remove introns.

  2. Add a cap (modified nucleotide) to the front of the 5’ end to protect it.

  3. Add a tail of adenine to prevent degradation and stabilise mRNA.

  4. This produces mature mRNA .

  5. This mRNA can be edited to create different versions of the mature mRNA, makes different proteins with different functions.

Post-translational gene expression:

Modify the polypeptides to make proteins of specific functions.

  1. Add lipoproteins or glycoproteins, involved in cell signalling.

  2. Modify amino acids to make bonds .e.g. change to cysteine to form disulphide bridges.

  3. Change how the protein folds, alter the tertiary or quaternary structure.

  4. Modification by cAMP + CRP (activation)

    CRP binds to cAMP which then binds to RNA polymerase to upregulate activity.

    cAMP activates kinesis which phosphorylates and activates other enzymes and proteins.

Homeobox genes:

  • Regulatory genes that code for part of a protein.

  • Control body development and the positioning of body parts.

  • Regulate mitosis and apoptosis.

Highly conserved in plants, animals and fungi. The same types of homeobox genes are found in all of them.

Mitosis- controls body plan, growth and development of cells, asexual reproduction.

Apoptosis- programmed cell death, to breakdown any unnecessary cells or tissues during development. They respond to external and internal stimuli (e.g. stress).

Hox genes:

Homeobox genes found in animals.

Highly conserved.

Regulatory genes that control body development and the positioning of body parts, the anterior and posterior body and they regulate mitosis and apoptosis.

The genes which regulate the cell cycle and apoptosis are able to respond to internal and external cell stimuli.