Cellular Control Flashcards
Define gene
- A section of DNA which codes for a particular polypeptide
- Heritable factor
- Occupies a specific position on a chromosome
How is differentiation of cells brought about?
- Expression of some genes in the genome
- Other genes are switched off
Define allele
- Various specific forms of a gene
What distinguishes an allele from a gene?
An allele is a variety of a gene
How are new alleles formed?
By mutation
How many chromosomes are found in human body cells (not including gametes)?
23 pairs i.e. 46 chromosomes
Define mutation
Random and spontaneous change in the base sequence of a gene
Identify different types of gene mutation
- Base substitution
- Insertion (causes frameshift)
- Deletion (causes frameshift)
Define frameshift
- A frameshift mutation is a genetic mutation caused by a deletion or insertion in a DNA sequence that shifts the way the sequence is read.
- Changes every successive codon from point of mutation
Explain why base substitutions don’t always result in a change in amino acid sequence
- Genetic code is degenerate
- Multiple codons code for same amino acid
Explain why a change in amino acid sequence can alter protein function
- Each amino acid has different R group
- R groups interact to determine tertiary structure of protein
- Different interactions can change protein shape
- Affects protein function
Describe the different possible effects of mutations
No effect
- No change to the phenotype of organism
- Functioning proteins still synthesised
Damaging
- Phenotype altered in negative way
- Functioning proteins no longer synthesised
- Can interfere with essential processes
Beneficial
- Protein synthesised results in new, useful characteristics
- Very rare occurrence
- e.g. mutation in proteins on CD40 cell surface membrane prevents HIV entry
What can cause mutations to occur?
Spontaneous mutation
- During DNA replication
Physical mutagens
- Break DNA strands
- e.g. ionising radiation
Chemical mutagens
- Chemically alter DNA bases
- e.g. deaminating agents
Biological agents
- Alter DNA sequence
- e.g. viral DNA inserted into genome
Define chromosome mutation
- Mutations that affect entire chromosome
- Not just one gene
Describe the different types of chromosome mutation
Deletion
- Section of chromosome breaks off and is lost
Duplication
- Section of chromosome duplicated
Translocation
- Section of one chromosome breaks off and joins another non-homologous chromosome
Inversion
- Section of chromosome breaks off and is reversed
Define gene expression
Gene expression is the translation and transcription of genetic information. It determines which genes are translated and transcribed and how many.
Outline the various ways that genes are regulated
Transcriptional
- Genes can be turned on or off
Post-transcriptional
- mRNA modified to regulate translation
Translational
- Translation can be stopped or started
Post-translational
- Proteins modified after synthesis to change function
Define heterochromatin
- DNA tightly wound around histones
- Chromosomes become visible under light microscope
Define euchromatin
- DNA loosely wound around histones
- Present during interphase
Why is transcription of heterochromatin not possible?
RNA polymerase cannot access genes
Explain why coiling of DNA around histones occurs
- DNA is negatively charged
- Histones are positively charged
Describe histone acetylation
- Addition of acetyl group to histone
- Acetyl groups decrease positive charge of histone
- DNA coils less tightly
Describe the effects of increased histone acetylation
- Histones are positively charged proteins, DNA is negatively charged
- Increased acetylation of histones decreases their positive charge, so bind DNA less tightly
- Transcription factors can access DNA
- Gene is switched on
Describe the effects of decreased histone acetylation
- Histones are positively charged proteins, DNA is negatively charged
- Decreased acetylation of histones increases their positive charge, so bind DNA more tightly
- Transcription factors can no longer access DNA
- Gene is switched off
Describe histone methylation
- Methyl (CH₃) group added to histone
- Makes histone more hydrophobic
- Binds to DNA more tightly
How does histone methylation decrease gene expression?
- Methylation is addition of a CH₃ group to histone
- Makes histone more hydrophobic
- Binds to DNA more tightly
- Prevents binding of transcription factors to DNA
Define epigenetics
- Study of changes in organisms caused by modification of gene expression rather than
alteration of the genetic code itself - Environmental factors can make changes to function of genes which can be inherited
How is gene transcription regulated at a DNA level?
By proteins that bind to specific base sequences in DNA
Define promoter
- Non-coding DNA with a function
- Binding site for RNA polymerase
- Controls gene expression
Explain how promoters control gene expression
- Every gene has a promoter immediately upstream of the coding sequence
- Base sequences vary
- Enables some genes to be transcribed, whilst others are not
Define operator
- Non-coding sequence in prokaryotes
- Binding site for repressor proteins and transcription factors
Define regulatory genes
Genes that make regulatory proteins
What are regulatory proteins?
- Proteins that bind to DNA sequences outside of promoter region
- e.g. transcription factors, repressor proteins
Define operon
- Group of genes that are under the control of the same regulatory mechanism
- Expressed at the same time
In which type of organisms are operons most common?
Prokaryotes
What is the advantage of a prokaryotic genome containing operons?
- Efficient
- If certain gene products not required, all of the genes involved can be switched off
What is the role of the lac operon?
- Group of genes in E. Coli
- Code for proteins that can metabolise lactose
- Switched on when glucose is not present
Define structural genes
- Genes that code for any protein product other than a regulatory factor
- e.g. enzymes, channel protein
Name the structural genes in the lac operon
- lacZ
- lacY
- lacA
Name the regulatory gene located near the lac operon
lacI
What does lacI code for?
- Repressor protein
- Prevents transcription of the lac operon
Explain how expression of the lac operon is inhibited
- Repressor protein constantly produced
- Binds to operator gene (lacO)
- Prevents RNA polymerase binding to promoter region
Explain the changes that occur when lactose is present
- Lactose binds to repressor protein
- Protein changes shape
- Can no longer bind to operator
- RNA polymerase binds to promotor
- Structural genes transcribed
- Enzymes synthesised
What type of gene regulation is the control of the lac operon?
Transcriptional
Explain the role of cAMP in the regulation of the lac operon
- cAMP binds to CRP (cAMP receptor protein)
- Increases rate of transcription of lac operon
Explain how an increase in glucose reduces transcription rate of the lac operon
- Glucose decreases level of cAMP
- Reduces level of active CRP
- Reduces transcription of lac operon
- If glucose present, it will be the preferred respiratory substrate
Describe the post-transcriptional control of gene expression
RNA processing
- Pre-mRNA spliced to remove introns
- Cap added to 5’ end
- Tail added to 3’ end
RNA editing
- Change in nucleotide sequence in mRNA
- Results in synthesis of different proteins
- Increases range of proteins that can be produced from a single gene
What is the role of the cap and tail in RNA processing?
- Stabilises mRNA
- Delay degradation in the cytoplasm
Outline the translational control of protein synthesis
Degradation of mRNA
- More resistant molecules last longer in cytoplasm
- So greater quantity of protein synthesised
Binding of inhibitory proteins to mRNA
- Prevents binding of mRNA to ribosomes
- Prevents protein synthesis
Activation of initiation factors
- Promote binding of mRNA to ribosomes
- Increases translation rate
Explain the role of protein kinases in translational control of protein synthesis
- Enzymes that catalyse addition of phosphate groups to proteins
- Changes tertiary structure
- Alters function or activates protein
- Protein kinases activated by cAMP
Outline the post-translational control of gene expression
- Addition of non-protein groups to proteins
- e.g. carbohydrate chains, lipids
- Modifying amino acids and formation of intermolecular bonds
- Folding or shortening proteins
- Modification by cAMP
- e.g. increasing rate of transcription of lac operon
Define morphogenesis
Regulation of the pattern of anatomical development
Define homeobox genes
- Regulatory genes
- All contain 180 base pair (bp) homeobox sequence
What type of protein do homeobox genes code for?
Regulatory protein
Which part of the protein does the homeobox sequence code for?
Homeodomain
- Section of protein that binds to DNA
What is the role of the homeodomain?
- Switches genes on or off
- Controls development of body plan
Define body plan
Position and development of body parts
Which organisms are homeobox genes found in?
- Plants
- Animals
- Fungi
Explain why homeobox genes are highly conserved across a wide range of organisms
- Genes are very important
- Mutation would alter body plan
- Many other genes would be affected
- Mutation likely to be lethal
- Would be selected against
Define Hox genes
- Group of homeobox genes only present in animals
- Found in gene clusters on different chromosomes
What is the role of Hox genes?
- Control correct positioning of body parts
- Order in which genes appear along chromosome is order which their effects are expressed
Define diploblastic animals
Animals with two primary tissue layers
Define triploblastic animals
Animals with three primary tissue layers
Define radial symmetry
- No left or right side
- Only top and bottom
- Typical in diploblastic animals
- e.g. jellyfish
Define bilateral symmetry
Organism have both left and right side and head and tail end
- e.g. humans
Define asymmetry
No lines of symmetry
- e.g. sponges
Outline the role of mitosis in development
- Increases number of cells
- Leads to growth
Define apoptosis
Programmed cell death
Outline the role of apoptosis in development
- Removes unwanted cells and tissues
- Reveals shape of body part
Outline the role of cell signalling in the shaping of organisms
- Cells undergoing apoptosis release chemicals
- Stimulate mitosis and cell proliferation
- Leads to remodelling of tissues
Which genes regulate mitosis and apoptosis?
Hox genes
Define stress
Condition produced when homeostatic balance within an organism is upset
Give examples of external factors that can cause stress
- Change in temperature
- Light intensity
Give examples of internal factors that can cause stress
- Hormones
- Psychological stress
Why can stress affect growth and development of animals?
Influences expression of Hox genes
