6.19 - Genetics of living systems

Question 1

base substitution mutation

Answer 1

• a base is changed during replication
• can be caused by a change in charge and pH
• least impactful mutation as code is degenerate and only one code is impacted (‘silent mutation’)

Question 2

insertion and deletion mutations

Answer 2

• a base is added or deleted during replication
• most impactful mutation as they are frame shift mutations, so affect every successive codon from point of insertion/deletion

Question 3

general effects of mutation

Answer 3

• most likely no effect on the phenotype of an organism because normally functioning proteins are still synthesised
• could be damaging if the phenotype of the organism is affected in a negative way (proteins no longer synthesised, or proteins synthesised are non-functional) so it interferes with one or more essential processes
• mutations being beneficial is very rare ( if it results in a new or useful characteristic such as a mutation in cell surface membranes that means HIV cannot bind, making some people immune to HIV)

Question 4

Physical mutagens

Answer 4

Ionising radiation such as X-rays
- can break one or both DNA strands
- some breaks can be fixed, but mutations can occur in the process

Question 5

Chemical mutagens

Answer 5

Deaminating agents
- chemically alter bases in DNA
- changes the base sequence

Question 6

Biological agents that increase the rate of mutation

Answer 6

Alkylating agents
- metal or ethyl groups are attached to bases
- results in the incorrect pairing of bases during replication
Base analogs
- molecules the same structure as bases
- incorporated into DNA in place of the usual base during replication, changing the base sequence
Viruses
- viral DNA may insert itself into a genome, changing the base sequence

Question 7

mutagen

Answer 7

a chemical, physical or biological agent that increases the rate of mutations

Question 8

Chromosome mutations

Answer 8

• genes altered in sections of chromosomes
• can be caused by mutagens and occurs during meiosis
  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 on the same chromosome

Question 9

Beneficial mutations

Answer 9

Lactose digestion
- relevantly recent mutation
- ability for humans to digest lactose after they cease to suckle
- found primarily in European populations
- could have prevented starvation and osteoporosis in famines

Sickle-cell anaemia
- caused by substitution in code for haemoglobin
- resistant to malaria as abnormally shaped erythrocytes
- malaria resistance outweighs negative symptoms, so is evolutionary beneficial

Question 10

amorphic

Answer 10

proteins doesn’t form, so is non-functioning/incomplete

Question 11

hypomorphic

Answer 11

reduction is protein function

Question 12

hypermorphic

Answer 12

increase in protein function (beneficial mutation)

Question 13

nonsense

Answer 13

no protein formed, code is non-functional
e.g. stop-codon substitution

Question 14

missense

Answer 14

incorrect sequernce/code

Question 15

need for regulation of gene expression

Answer 15

• growth and development
• different life stages e.g. caterpillar, butterfly
• respond to changes
• short responses such as enzymes and hormones
• synthesis demands

Question 16

stages where gene expression is regulated

Answer 16

Transcriptional
- Genes can be turned on or off
Post-transcriptional
- mRNA can be modified
- Regulates translation and the types of proteins produced
Translational
- Translation can be stopped or started
Post-translational
- Proteins can be modified after synthesis
- Changes the function of proteins

Question 17

Operon

Answer 17

a group of genes that are under control of the same regulatory mechanism and are expressed at the same time. Far more common in prokaryotes than eukaryotes

Question 18

Lac operon

Answer 18

• a group of 3 genes involved in the metabolism of lactose
• structural genes are they code for 3 enzymes
• transcribed onto a single long molecule of mRNA
• a regulatory gene is located near to the operon that codes for a repressor protein
• the repressor protein prevents the transcription of the structural genes in the absence of lactose
• lactose binds to the repressor protein, changes its shape so it can no longer bind to the operator
• as a result, RNA polymerase can bind to the promoter, so the genes can be transcribed and the enzymes synthesised

Question 19

Chromatin remodelling as a form of transcriptional control

Answer 19

Chromatin - DNA wound around histone proteins in order to be packed into the nucleus of a cell
Euchromatin - loosely wound DNA present during interphase
Heterochromatin - tightly wound DNA causing chromosomes to be visible during cell division
The genes in Euchromatin can be freely transcribed, but cannot in heterochromatin as it is too tightly wound.
- simple form of regulation that ensures all proteins needed for cell division are synthesised in time and prevents protein synthesis from happening as cell is dividing (as synthesis is complex and energy consuming)

Question 20

Histone modification

Answer 20

• DNA coils around histones as histones are positively charged and DNA is negatively charged
  Methylation
• methyl groups are added to DNA and histones
• causes histones to become more hydrophobic
• histones pack more tightly together to avoid aqueous external environment
• DNA coils more tightly so transcription factors cannot bind to DNA and genes are not expressed
  Acetylation
• addition of acetyl groups (phosphorylation) reduces positive charge of histones
• causes DNA to coil less tightly as is less attracted to histones
• allows certain genes to be transcribed as transcription factors are able to bind to DNA

Question 21

post-transcriptional control, pre-mRNA

Answer 21

• pre-mRNA is modified, forming mature mRNA
• a cap (modified nucleotide) is added to the 5’ end
• a tail (long chain of adenine nucleotides) is added to the 3’ end that helps to stabilise mRNA and delay degradation in the cytoplasm
• RNA is cut at specific points (splicing)
• introns (non-coding DNA) are removed
• exons (coding DNA) are joined together

Question 22

translational control, degradation of mRNA

Answer 22

• regulates process of protein synthesis
• the more resistant the molecule the longer it will last in the cytoplasm, so a greater quantity of protein synthesised.
• Depends on how molecule is processed

Question 23

translational control, inhibitory proteins

Answer 23

• regulates process of protein synthesis
• binding of inhibitory proteins to mRNA prevents it from binding to ribosomes and the synthesis of proteins
• binding of inhibitory proteins to ribosomes

Question 24

translational control, initiation factors

Answer 24

• regulates process of protein synthesis
• activation of initiation factors which aid binding of mRNA to ribosomes
• e.g. eggs of many organisms produce large quantities of mRNA which is not required until after fertilisation when initiation factors are activated

Question 25

translational control, protein kinases

Answer 25

• regulates process of protein synthesis
• enzymes that catalyse the addition of phosphate groups to proteins
• changes tertiary structure and therefore the function of the protein
• many enzymes are activated by phosphorylation
• kinases often activated by secondary messenger cAMP

Question 26

post-translational control, non-protein groups

Answer 26

• modifications to proteins that have been synthesised
• addition of non-protein groups
  e.g. carbohydrate chains, lipids, phosphates

Question 27

post-translational control, amino acid modification

Answer 27

• modifications to proteins that have been synthesised
• modifying amino acids
• formation of bonds such as disulfide bridges that alters proteins structure and function

Question 28

post-translational control, tertiary structure

Answer 28

• modifications to proteins that have been synthesised
• folding or shortening of proteins affects structure

Question 29

post-translational control, cAMP

Answer 29

• modification to proteins that have been synthesised
• modified by cAMP

Question 30

role of cyclic AMP (cAMP)

Answer 30

• binding of RNA polymerase still only results in a relatively slow rate of transcription that needs to be increased or up-regulated to produce the desired quantity of proteins
• this is achieved by the binding of cAMP receptor protein (CRP), which is only possible if cAMP is bound to CRP

Question 31

morphogenesis

Answer 31

the regulation of the pattern of anatomical development

Question 32

homeobox genes

Answer 32

• a section of DNA 180 base pairs long
• regulatory genes
• control body development
• highly conserved in plants, animals, fungi
• regulate mitosis and apoptosis
• respond to internal and external stimuli

Question 33

hox genes

Answer 33

• the group of homeobox genes that are only present in animals
• responsible for correct positioning of body parts
• found in gene clusters (4 on different chromosomes)

Question 34

hox genes leading to different complexities of animals

Answer 34

• asymmetry e.g. seas sponges
• radial symmetry (along central axis) e.g. jellyfish
• bilateral symmetry (across middle) e.g. humans, insects

Question 35

ontogeny

Answer 35

embryonic development

Question 36

apoptosis

Answer 36

• programmed cell death
• useful for when cells get old
• sculpts body parts
• removal of tissues or organ sculpting
• regulated by hox genes

Question 37

factors affecting the expression of regulatory genes

Answer 37

• internal and external environment
  internal (release of hormones, psychological stress)
  external (change in temperature, intensity of light)
• these factors have a greater impact during the growth and development of an organism
• drugs
  e.g. Thalidomide given to pregnant women in 1950s-60s prevented normal expression of a hox gene which resulted in babies born with shorter limbs