Cellular control (6.1.1)
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There are multiple types of gene mutations. In substitution (point) mutations, one DNA base is replaced by another during DNA replication. There are three different types of substitution mutation:
- Silent: the base sequence changes, but the same amino acid is coded for, so the protein is unchanged.
- Missense: the base substitution changes the codon, resulting in a different amino acid being incorporated into the protein. This may alter the protein’s structure and function.
- Nonsense: the base substitution creates a stop codon, causing translation to end prematurely and producing a shortened, usually non-functional protein.
In insertion / deletion (indel) mutations, one or more bases are added or removed. Unless the number of bases inserted or deleted is a multiple of three, this causes a frameshift mutation altering the triplet reading frame.
As a result, amino acid sequence from the mutation onwards is changed, often producing a protein with an altered tertiary structure that is unable to function normally.
Gene expression can be controlled at the transcriptional level. During differentiation, some genes are switched on while others are switched off.
Expressed genes are transcribed into mRNA and translated into proteins. These proteins determine cell structure and function by controlling biochemical processes (e.g., enzymes and hormones) and forming structural components (e.g., cytoskeletal and membrane proteins).
The lac operon is a group of genes in the bacterium E. coli, involved in bacterial lactose metabolism. It is an example of gene regulation at the transcriptional level in prokaryotes.
The enzymes that break down lactose into smaller sugars are only produced when lactose is present:
- When lactose is absent a repressor protein binds to the operator preventing RNA polymerase from transcribing the genes.
- When lactose is present, it binds to the repressor, causing a shape change. The repressor detaches from the operator, allowing RNA polymerase to bind to the promoter region and move past the operator to the genes. The genes are transcribed into mRNA, and enzymes to digest lactose are produced.
Gene expression can be controlled at the post-transcriptional level.
Introns are non-coding regions of a DNA within genes, while exons are the coding regions.
During transcription both introns and exons are transcribed to produce primary mRNA which is then edited to remove the introns. The exons are then spliced together producing mature mRNA that can be translated into a protein.
Gene expression can be controlled at the post-translational level.
A large number of proteins, including enzymes are activated or inactivated by phosphorylation.
Cyclic AMP (cAMP) is a second messenger that activates protein kinase A (PKA) which phosphorylates target proteins, altering their activity.
PKA can also phosphorylate the CREB protein which can enter the nucleus and act as a transcription factor, regulating gene expression.
The homeobox regulates gene expression associated with anatomical development in plants, animals and fungi. It is a sequence of 180 DNA base pairs, which encodes a 60 amino acid sequence.
These gene sequences are highly conserved and very similar across species.
Hox genes are only found in animals and are a subset of the homeobox genes.
Hox genes regulate embryonic development and control where body parts grow along the anterior-posterior axis. Mutations in Hox genes can cause limbs to grow in the wrong locations.
Hox genes encode for proteins that act as transcription factors to switch other genes on or off, regulating processes such as cell division, apoptosis and cell migration.
Mitosis is a tightly regulated process that increases cell number during growth and development, helping to establish and maintain body form.
Genes that regulate the cell cycle respond to internal signals (e.g., DNA damage and cyclin levels) and external signals (e.g., growth factors) to determine whether a cell divides.
During embryonic development, homeobox and Hox genes regulate where and when mitosis occurs, ensuring cells divide in the correct locations to form tissues and organs.
Apoptosis is programmed cell death. It is vital in the development of both plant and animal tissues and can be triggered by internal stimuli such as DNA damage inside the cell, or by external stimuli such as signals from other cells.
Apoptosis occurs in several steps:
- The signal to self destruct is received.
- Enzymes are activated and break down the cytoskeleton.
- Small protrusions known as blebs form on the cell surface membrane.
- Chromatin condenses, the nucleus breaks apart and DNA is fragmented.
- The cell fragments into smaller vesicles, so that the cell debris can be engulfed by phagocytes.