genetics of living systems Flashcards
1
Q
What is a mutation?
A
A change in the sequence of bases in DNA • Caused by substitution, deletion or insertion of one or more nucleotides within a gene • If only 1 nucleotide is affected, it is called a point mutation • Insertion or deletion leads to frameshift mutation (unless the number of nucleotides changes is multiple of 3)
2
Q
What is a frameshift mutation?
A
The addition or deletion of a nucleotide moves, or shifts, the reading frame of the sequence of bases. This will change every successive codon from the point of mutation
3
Q
What are the effects of
different mutations?
A
No effect - because normally functioning proteins are still synthesised • Damaging - phenotype is affected negatively because proteins are no longer synthesised or synthesised proteins are non functional • Beneficial - protein is synthesised that results in a new and useful characteristic in the phenotype
4
Q
What are mutagens?
A
A chemical, physical or biological
agent which causes mutations
5
Q
What are the main mutagens?
A
6
Q
What is the difference between
gene mutations and
chromosome mutations?
A
Gene mutations occur in single genes or sections of DNA, whereas chromosome mutations affect the whole chromosome, or a number of chromosomes within a cell
7
Q
What causes chromosome
mutations?
A
They can be caused by mutagens
and normally occur during meiosis
8
Q
What types of changes can
occur in chromosome
structure?
A
• Deletion - a section of chromosome breaks off and is lost within the cell • Duplication - sections get duplicated on a chromosome • Translocation - a section of one chromosome breaks off and joins another non-homologous chromosome • Inversion - a section of chromosome breaks off, is reversed, and then joins back onto the chromosome • Non-disjunction - one pair of chromosomes fails to separate, leaving one gamete with an extra chromosome e.g. down syndrome or trisomy
9
Q
How are genes regulated?
A
The ways they are regulated is categorised by the level at which they operate • Transcriptional - genes can be turned on or off • Post-transcriptional - mRNA can be modified which regulates translation and the types of proteins produced • Translational - translation can be stopped or started • Post-translational - Proteins can be modified after synthesis which changes their functions
10
Q
How is DNA stored?
A
It is wound around proteins called histories in order to be packed into the nucleus of a cell The resulting DNA/protein complex is called chromatin
11
Q
What is the difference between
heterochromatin and
euchromatin?
A
• Heterochromatin is tightly wound DNA, causing chromosomes to be visible during cell division • Euchromatin is loosely wound DNA present during interphase
12
Q
Why does protein synthesis not
occur during cell division?
A
• Heterochromatin is present during cell division • Transcription of genes is not possible when DNA is tightly wound because RNA polymerase cannot access the genes • The genes in euchromatin can be freely transcribed, so protein synthesis can occur during interphase
13
Q
What is the benefit of protein
synthesis not occurring during
cell division?
A
• Ensures the proteins necessary for cell division are synthesised in time • Prevents the complex and energyconsuming process of protein synthesis from occurring when cells are actually dividing
14
Q
Why does DNA coil around
histones?
A
Histones are positively charged and
DNA is negatively charged
15
Q
Why are histones modified?
A
To increase or decrease the degree of packing (or condensation)
16
Q
How are histones modified?
A
• Acetylation and phosphorylation reduce the positive charge on the histones (making them more negative), causing DNA to coil less tightly so certain genes can be transcribed • Methylation makes the histones more hydrophobic so they bind more tightly to each other, causing DNA to coil more tightly and preventing the transcription of genes
17
Q
What is epigenetics?
A
External control of genetic regulation
18
Q
What is an operon?
A
A group of genes that are under the control of the same regulatory mechanism and are expressed at the same time • Very efficient way of saving resources because if a certain gene products are not needed, then all of the genes involved in their production can be switched off
23
Q
How is transcription of the
structural genes prevented in
the absence of lactose?
A
1. A regulatory gene, I, is located near to the operon and codes for a repressor protein (LacI) 2. When this gene is expressed, the repressor protein produced binds to to the operator, preventing RNA polymerase from binding to the promoter region 3. The repressor protein therefore prevents the genes lacZ and lacY from being transcribes 4. The enzymes for lactose metabolism are not made 5. The genes are “off”
29
Q
Describe post-transcriptional
gene regulation
A
1. All the DNA of a gene, both introns and exons, is transcribed. The resulting mRNA is called primary mRNA 2. Primary mRNA is then edited, and the RNA introns (lengths corresponding to DNA introns) are removed 3. The remaining mRNA exons (corresponding to the DNA introns) are joined together 4. Endonuclease enzyme may be involved in the editing and splicing process 5. A length of DNA with its introns and exons can, according to how it’s spliced, encode more than one protein
33
Q
How is cAMP used in post
translation gene expression
regulation?
A
1. A signalling molecules e.g. glucagon, binds to a receptor on the plasma membrane of the target cell 2. This activates a transmembrane protein which then activates a G protein 3. The activated G protein activates adenyl class enzymes 4. Activated adenyl cyclase enzymes catalyse the formation of many molecules of cAMP from ATP 5. cAMP activates PKA (protein kinase A) 6. Activated PKA catalyses the phosphorylation of various proteins hydrolysing ATP in the process. This activated many enzymes in the cytoplasm e.g. those that convert glycogen into glucose 7. PKA may phosphorylate another protein (CREB, cAMP response element to binding) 8. This then enters the nucleus and acts as a transcription factor to regulate transcription
42
Q
What is the common feature of
animals?
A
They are segmented e.g. the rings of a worm or the back none of vertebrates • These segments have multiplied over time and are specialised to perform different functions
46
Q
What are the sequence of
events during apoptosis?
A
1. Enzymes break down the cell cytoskeleton 2. The cytoplasm becomes dense with tightly packed organelles 3. The cell surface membrane changes, and small protrusions called blebs form 4. Chromatin condenses, the nuclear envelope breaks and DNA breaks into fragments 5. The cell breaks into vesicles that are ingested by phagocytic cells, so that cell debris does not damage any other cells or tissues. The whole process happens quickly • Apoptosis is controlled by cell signalling • Some signalling molecules 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 e.g. stress • Signalling molecules include: cytokines, hormones, growth factors and nitric oxide
