6.1: Cellular Control Flashcards
What is a gene?
A length of DNA that codes for a polypeptide or a length of RNA that is involved in regulating gene expression
What is a mutation?
Random change in DNA base sequence caused by mutagens
What is a point mutation?
One base pair replaces another
What are the three types of point mutations?
- Silent mutation
- Misense mutation
- Nonsene mutation
What is a silent mutation?
One of the bases change
No change to polypeptide and it has no effect due to the degenerate code of DNA
This means it produces the same amino acid
What is a misense mutation?
- Change in amino acid sequence
- Changes the primary, secondary and tertiary structure
- Changes the polypeptide
- Degenerate code has failed and cannot carry out usual function
What is a nonsense mutation?
A codon changes to a STOP codon
Results in a truncated protein that will not function
What is an indel mutation?
One or more nucleotides are inserted/deleted from a length of DNA
What happens in an indel mutation?
A base is inserted or deleted and the base triplets are altered
A frameshift occurs
Due to the bases being read as non overlapping
Primary and tertiary structure is altered
Protein cannot carry out its normal function
What is a beneficial mutation?
Helped to drive evolution through natural selection
What are two examples of benefical mutations?
Mutation of blue eyes:
- Helps people see bettwe in less bright light in temperate zones
- Dangerous in areas where sunlight intensity is higher - lack of iris pigmentation could lead to lens cataracts
Melanin:
- High concentration of melanin protected early humans in Africa from sunburn and skin cancer
- In temperate conditions paler skin would be an advantage enabling vitamin D to be made with a lower intensity of sunlight - protects form heart disease and cancer
What are neutral mutations?
Neither harmful or benefical
What are two examples of neutral mutations?
- Inability to smell different flowers
- Differently shaped ear lobes
What happens in glucose is absent in E. coli and lactose is present?
E. coli normally metabolises glucose as a respiratiory substrate
When it is not present and lactose is present it induces the production of two enzymes:
- Lactose permease
- Beta galactosidase
What does lactose permease do?
Allows lactose to enter the bacterial cell
What does beta galactosidase?
Hydrolyses lasctose to glucose and galactose
What is the lac operon and what does it consist of?
Where gene expression is regulated in E. coli
Consists of:
- Regulatory gene - codes for repressor protein
- Promoter region
- Lac O - length of DNA containing an operator region
- Structural genes lac Z and Lac Y - code for beta galactosidase and lactose permease
What happens when lactose is not present?
4 steps
- Regulatory gene is expressed and produces repressor protein
- This binds to operator region, preventing RNA polymerase from binding to the promotor region
- Prevents lac Z and lac Y from being transcribed
- Enzymes for lactose metabolism is not made
What happens when lactose is present?
4 steps
- Lactose binds to repressor gene
- Alters the shape of the repressor protein and prevents it from binding to the operator
- RNA polymerase can bind to the promotor region and start transcribing the structural genes into mRNA
- This wil then be translated into the two enzymes
What are transcription factors?
A protein or region of short non coding RNA that can combine with a specific site on a length of DNA and inhibit/activate transcription of the gene e.g repressor protein
What are introns?
Non-coding regions of DNA
What are exons?
The coding o expressed region of DNA
What is an example of post transcriptional gene regulation?
Alternative splicing
What happens in alternative splicing?
4 steps
- Both introns and exons are transcribed (primary mRNA)
- Primary mRNA is edited and RNA introns are removed
- The remaining mRNA exons are joined together
- Endonuclease enzyme may be involved in the editing and splicing process
What is an example of post translational level of gene regulation?
Enzyme phosphorylation
What is enzyme phosphorylation?
8 steps
- A signalling molecule binds to a receptor on the plasma membrane of the target cell
- Activated a transmembrane protein which then activates a G protein
- Activated G protein activated adenyl cyclase enzymes
- Activated adenyl cyclase enzymes catalyse the formation of many molecules of cAMP form ATP
- cAMP actiavtes PKA (protein kinase A)
- Activated PKA catalyses the phosphorylation of various proteins, hydrolysing ATP in the process. This phosphorylation activates many enzymes in the cytoplasm
- PKA may phosphoylate another protein (cAMP, CREB response element binding)
- This then enters the nucleus and acts as a transcription factor to regulate transcription
What are hox genes
also referred to as homeobox genes in plant, animal and fungi kingdoms
Control the development of body plans
They code for polypeptides that act as transcription factors at different stages throughout development
What is colinearity?
Sequence of genes on the chromosome = sequence of body parts (and time of expression)
What are homeobox sequences/regions?
Hox genes contain a homeobox sequence
- Sequence = 180 base pairs found within genes
- Involved in regulating patterns of anatomical development in animals, fungi and plants
- Highly conserved in plants, animals and fungi
What are homeobox proteins?
Act as transcription factors
Can change:
- Rate of cell cycle
- Apoptosis
What are homeodomains?
Coded by homeobox regions
- HTH (helix turn helix) - good at binding to DNA
- A 60 amino acid sequence forms a polypeptide involved in DNA binding
What is apoptosis?
Programmed cell death e.g for development
Can be triggered in response to other things:
- Intracellular signals such as DNA damage - mitosis checkpoint has failed
- External signals such as t killer cells
What occurs in apoptosis?
5 steps
- Enzymes break down the cytoskeleton
- The cytoplasm becomes dense with tightly packed organelles
- The cell surface membrane changes and small protrusions called blebs form
- Chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments
- The cell breaks into vesicles that are ingested by phagocytic cells, so that cell debrs does not damage any other cells or tissues
How is apoptosis controlled?
Cell signalling molecules that may be released by cells when genes that are involved in regulating the cell cycle and apoptosis respond to internal cell stimuli and external stimuli such as stress
For example:
- Nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to hydrogen ions and dissipating the proton gradient
- Proteins are released into the cytoplasm where they bind to apoptosis inhibitor proteins, allowing apoptosis to occur
What happens in too little or to much apoptosis
Too little:
Leads to formation of tumours
Too much:
Leads to cell loss and degeneration (Alzhiemer’s)
