Chapter 19 Genetics Flashcards
What is a gene? What is the difference between a gene and allele?
A base sequence which codes for a particular protein.
Genes are the loci for the protein, the same in all organisms of the species, whilst alleles are the different versions of the base sequence
What is a mutation? What are the 3 main types of mutation that occur?
A change in the base sequence
Substitution, Insertion, Deletion
What is substitution? What type of mutations can result and what happens with each?
When one nucleotide is replaced by another
Nonsense- replaced by a stop codon, stops transcribing
Mis-sense- different amino acid coded, so protein may or may not work
Silent- same amino acid coded for but a different codon, as code degenerate
Why would 7 codons only code for 6 amino acids?
The final codon is the stop codon, this doesn’t code for an amino acid
What are insertion and deletion mutations? What do they cause?
Insertion- an extra nucleotide is added
Deletion- a nucleotide is removed
Frameshift mutations- all further amino acids affected, including changing start and stop codons
What are the 3 main effects of mutations to humans? Explain how they arise?
Neutral- normal functioning occurs, unchanged phenotype
Harmful- non-function protein or not synthesised, changed phenotype
Beneficial- protein, more suitable for function, synthesised, changed phenotype
What are the main categories and examples of mutagens?
Physical, breaks apart the DNA strands- X-rays, gamma, UV
Chemical, deamination and C to U- Cigarette smoke, nicotine, tar, caffiene
Biological, alter DNA- Alkylating agents, viruses
What is the difference between chromosomal and point mutations?
Point- insertion/deletion/substitution- only affect a few nucleotides/ caused by a change in a nucleotide, so a gene
Chromosomal- affects the whole chromosome rather than a single gene
What are examples of chromosomal mutations?
Translocation- section breaks off, joins another non-homologous
Inversion- section breaks off, reverses, rejoins
Deletion, duplication
What is the process of transcription?
DNA helicase catalyses the breakdown of hydrogen bonds between nucleotides of a certain gene, the gene unzips and unwraps
RNA polymerase polymerase binds to the DNA strand
Free mRNA nucleotides form complimentary base pairs with anti-sense strand, and as RNA polymerase moves along the DNA, it catalyses the formation of phosphodiester bonds in the mRNA
It also enables the section of DNA behind it to rejoin
This repeats until the stop codon is reached
What is the idea behind epigenetics?
Each cell has the whole genome, but only certain genes are activated
This can be controlled, and genes and can switched on and off, which has led to cell differentiation
What are the main types of cellular control?
Transcriptional- transcription factors and operons, chromatin
Post-transcriptional- exons/introns
(Post)translational- PKA, Golgi body, inhibition of translation
What are transcription factors? How do things work when a gene is activated?
Transcription factors- proteins which move in from the cytoplasm and attach to the DNA to attach RNA polymerase to the DNA
Normally, the transcription factor will bind to the DNA at the operator, then RNA polymerase binds at the promoter and transcription occurs
What happens to transcription when a gene is switched off?
A repressor molecule will bind to the operator or to the transcription factor, preventing the transcription factor from binding to the DNA
This means RNA polymerase cannot attach to the promoter
Transcription of that gene cannot occur, gene switched off
How can genes be turned on, based on what happens to transcription factors?
A molecule can either be synthesised by another gene, or a hormone can be secreted and sent to the cell
This molecule can bind to the repressor molecule, causing a conformational change, which releases the repressor from the TF or DNA
The TF can now bind to the DNA at the operator, and enable RNA polymerase to bind to the promoter, thus allowing transcription of the gene
What is an operon?
A section of DNA that contains a cluster of genes that are all transcribed together, as well as control genes and a regulatory gene
Most common in prokaryotes
What is a regulatory gene? What is the operator? What is the promoter? What is a structural gene?
Regulatory- continually synthesises a repressor protein
(Operator- site on DNA where the repressor will bind, preventing the transcription factor joining)
Promoter- site on DNA where RNA polymerase can bind
Structural- contain the base sequence for a useful protein
What is happening with the Lac Operon?
The regulatory gene continually synthesises a repressor protein
This will bind to the operator, preventing a TF from binding, and so RNA polymerase cannot bind and transcription cannot occur
When lactose is present, it will bind to the repressor, causing a conformational change that releases the repressor, it cannot bind to the operator
The TF can now bind to the operator, and RNA polymerase to the promoter, enabling synthesis of the structural gene, to break down lactose
What is an activator? What is a repressor?
Activator- starts transcription (e.g a transcription factor binding to the operator)
Repressor- stops transcription by blocking the binding site of the TF and the DNA (e.g repressor synthesised from regulatory gene)
What are exons and introns?
Exons- code for an amino acid
Introns- does not code for an amino acid despite being part of a gene
What is post-transcriptional cellular control?
Editing primary (pre) mRNA by removing the introns, leaving only exons forming mature mRNA
A cap is added to the 5’ end, and tail of adenine to the 3’ to stabilise the mRNA and delay degradation
Modification of the mRNA sequence when removing the introns
What is the process of pre mRNA being converted into mature mRNA?
snRNP protein complex causes a spliceosome of introns to form
The lariat of introns are excised and the exons are spliced together
The mRNA can then leave the nucleus via the nuclear pore
What are examples of translational control?
Degradation of mRNA
Inhibitory proteins to mRNA
Initiation factors
What are examples of post-translational control?
Activation by PKA, via phosphorylation, triggered by cAMP as secondary messengers, controlled by hormones
Modifications to proteins are the golgi apparatus, e.g adding disulphide bridges and carbohydrate chains
Changing transcription levels by cAMP
What is heterochromatin and euchromatin? How does this affect transcription?
Heterochromatin- tightly wound histones during cell division, RNA polymerase cannot bind to DNA, so no transcription
Euchromatin- loosely wound histones, during interphase, so RNA polymerase can bind and transcription is possible
How can histones be modified to turn genes on and off?
DNA negative from phosphates, histones positive
Methylation of Histones- more hydrophobic, so greater interaction with DNA, more packing, unable to transcribe
Acetylation of Histones- histones less positive, less attraction, less tight, so RNA polymerase can bind and transcription can occur
Control= Epigenetics
How do hox genes and homeodomains help regulate development?
Hox genes have homeobox sequences which code for homeodomains
The homeodomains are contained within proteins, and the complex can act as a TF
The TF can either repress or activate the transcription of the developmental genes
This often includes synthesis of a molecule which can then causes mitosis or apoptosis of the cell or other cells, and cell differentiation
How many clusters of homeobox genes do humans and flies have?
Humans have 4 clusters
Flies have 1 cluster
What is apoptosis?
A series of biochemical events leading to an orderly and tidy cell death
Where is apoptosis used?
To remove senescent cells which have a higher likelihood of becoming cancerous
Definition of tissues- bulk masses, but apoptosis gives shape e.g with hands
How are apoptosis and mitosis linked? What happens when the ratio changes?
Mitosis > Apoptosis, tumour
Mitosis < Apoptosis, cell loss and degeneration
What type of genes regulate mitosis and apoptosis and how?What regulates expression of these?
Hox genes- code for homeodomain proteins which can act as transcription factors for apoptosis and mitosis triggering genes
Stresses and stimuli
What are examples of internal and external stresses for cell regulation?
Internal- damage to DNA, hormones, psychological stress
External- light/temp, pathogens, lacking nutrients, drugs
What is the process of apoptosis?
The cell begins to bleb and the nucleus starts to disintegrate
Hydrolytic enzymes causes the formation of cell fragments, with intact plasma membranes and organelles
Cell fragments are ingested and digested by phagocytic cells
What do you need to specify when suggesting new alleles, genes? E.g the cause of a new characteristic is caused by which type of gene?
Whether it is due to a structural gene modification or regulatory
Epistasis
What are homeotic genes? What are homeobox genes?
Homeotic Genes: genes that organise the shape of organisms
Homeobox Genes: a subset of homeotic genes that contain the homeobox
What is the homeobox?
A small section of 180 base pairs found within a homeobox gene which codes for a homeodomain
This region is highly conserved in all organisms of different kingdoms
How do homeobox gene related to homeodomains?
The homeobox gene will code for a certain protein
Part of this gene is the homeobox, which codes for a homeodomain, helicles-turn-helicles
The homeodomain is part of this entire protein, and is what enables the protein to act as a transcription factor for nearby genes
What are hox genes and how do they function?
A type of homeobox gene which is only found in animals. They position body parts in an organism
They still contain a homeobox sequence so code for a homeodomain
They act as transcription factors for genes which control mitosis, apoptosis, and cell differentiation
How are hox genes organised?
Spatial linearity: the hox genes that code for the head will be grouped together, and as you move down the chromosome, the hox genes will code for the body and at the end the tail
Position on chromosomes mirrors position with body length
Temporal Linearity:
Head hox genes expressed first, then body, then tail
The order in which the hox genes are transcribed is the same as their position down the chromosomes
Are hox genes highly conserved? What overall do they synthesise?
Yes, the whole gene as well as the homeobox
Code for a polypeptide
Part of this peptide is a TF, but it is still a polypeptide
How may a mutation not change the protein produced?
If it has changed, what is the likely type of mutation?
Silent mutation, so same amino acid, so amino acid sequence, same protein
Or changing one protein does not alter the secondary/tertiary structures enough to alter the function of the protein
Or the mutation is in an intron
If changed, likely a frameshift with a downstream effect on multiple amino acids, and possibly stop codons
What does OCR rly want me to know about regulation after transcription?
Primary mRNA converted to mature mRNA via the removal of introns, leaving only exons
Snarp proteins form a complex and a spliceosome to excise the introns
Alternative splicing can take place to produce different versions of the mature mRNA
cAMP may need to activate the proteins synthesised by binding to them causing conformational changes
Why are fruit flies used to investigate hox genes?
Short life cycles
Low cost
Fruit fly genetics already well understood, so easier to make and then understand the change
Simpler genetics
What can be concluded about the time elapsed since a common ancestor if the species have similar hox genes?
Not much
Hox genes highly conserved within all organisms
