Genetics of living systems

Question 1

State the types of mutagens

Answer 1

o Chemical: chemically alter bases
o Deaminating agents e.g. mould toxin liver cancer
o Physical: breaks DNA strands
o Ionising radiation e.g. X-ray, UV, genome
o Biological:
o Alkylating elements: add methyl groups to bases as base pairs are incorrect e.g. chemotherapy drugs
o Viruses: viral DNA added to genome e.g. HPV and cervical cancer
o Base analogs: added to DNA in place of normal bases and change the base sequence e.g. research avenue for cancer

Question 2

Describe what point mutation is and the different types

Answer 2

o Changes in an individual gene due to miscopying of one or more nucleotides
o Deletion of insertion of a nucleotide results in a frameshift
o Substitution mutation results in a change of one base
o Silent: doesn’t affect overall structure of protein
o Misense; sequence makes wrong protein
o Nonsense: sequence makes no protein

Question 3

What are ETNRs

Answer 3

o Some genes have a repeating triplet

o In every ETNR the no. repeats increases over every generation at meiosis

Question 4

Whats chromonsomal mutation and what are the types

Answer 4

o Gene depletion: gene removed
o Gene duplication: gene repeated
o Inversion: gene turned upside down
o Translocation: one chromosome switches with another chromosome
o Whole chromosomal mutation: an entire chromosome is lost or repeated during cell division e.g. down syndrome caused by extra chromosome 21

Question 5

What are transcription factors

Answer 5

o Transcriptional factors: proteins or non-coding sections of RNA which control which genes are switched on or off
o Seek and bind to promotor regions which aid or prevent the binding of RNA polymerase

Question 6

Describe what post transcription control is

Answer 6

o Within a gene there are many non-coding (introns) and coding (exons) sections
o Both are transcribed producing primary mRNA
o This is edited and introns are removed

Question 7

Describe translational control

Answer 7

o mRNA degradation rate can affect the quantity of proteins produced
o Inhibition of mRNA binding to ribosomes
o Activation of initiation factors which aid mRNA binding to ribosomes

Question 8

Describe post-translational control

Answer 8

o Involves activation of proteins
o Phosphorylation
o Modification of amino acids and bonds in tertiary structure
o Modification of cAMP.

Question 9

Describe the process of protein activation

Answer 9

o A signalling molecule binds to a receptor on the cells plasma membrane
o Transmembrane protein is activated which activates a G protein
o Activated G protein activates adenyl cyclase enzymes
o Activated adenyl cyclase catalyses the formation of lots of cAMP from ATP
o cAMP activates protein kinase A (Pka)
o Activated protein kinase catalyses the phosphorylation of various proteins hydrolysing ATP in the process. This phosphorylation activates many enzymes in the cytoplasm e.g. that convert glycogen to glucose
o Pka may phosphorylate another protein (CREB = cAMP response element binding)
o CREB then enters the nucleus and acts as a transcription factor to regulate transcription

Question 10

Describe how an embryo develops (in relation to body plans)

Answer 10

o Fertilised egg has no body plan

o Development of a body plan starts very early in development and is similar in all organisms

Question 11

Describe how drosophila melanogaster are used

Answer 11

o Most knowledge of body plan comes from fruit flies
o Good model organism because:
o Small
o Short life span
o Cheap to look after and get
o High breeding rate
o Comes from same kingdom as humans etc.
o Some studies suggest body plan development is controlled by a set of genes called homeobox genes
o Homeobox gene code for proteins that act as transcription factors containing homeodomains, that bind to DNA and switch genes on and off

Question 12

What are the types of homeobox gene

Answer 12

o Maternal effect genes: establish body polarity (anterior/posterior, dorsal/ventral)
o Segmentation genes: determine segments of genes (head/thorax/abdomen)
o Homeotic genes: control development of the individual body segments (wings in right place along thorax/eyes develop on head segment

Question 13

Describe the risks of mutations in home genes

Answer 13

o Mutations can lead to some very profound deformities in organisms
o Some parts may be missing or develop in the wrong place

Question 14

Describe the features of homeobox genes

Answer 14

o Found in all living organisms and are remarkably highly conserved (show little difference in base sequence)
o 180 base pairs long and code for proteins – most of which are transcription factors
o Homeobox genes are arranged in clusters called hox clusters
o Worms have 1 hox cluster
o Drosophila have 2 hox clusters on one chromosome
o Vertebrates have 4 hox clusters, each on a separate chromosome

Question 15

Describe the different layouts of living organisms with examples

Answer 15

o Beetles have bilateral symmetry
o Coral polyp has radial symmetry
o Sponge has no symmetry

Question 16

What is an operon

Answer 16

group of genes that are under control of the same regulatory mechanism and are expressed at the same time

Question 17

What is the lac operon and describe what happens

Answer 17

o Lac operon: group of three genes lacZ, lacy, lacA. They are all structural genes as they code for three enzymes and are transcribed onto a single long molecule of Mrna.
o A regulatory gene, lacI, is located near to the operon and codes for a repressor protein that prevents the transcription of the structural genes in the absence of lactose
o The repressor protein is constantly produced and binds to the operator
o Binding of this protein prevents RNA polymerase binding to DNA and beginning transcription which is called regulation. The section of DNA that is the binding site for RNA polymerase is called the promoter
o When lactose is present, it binds to the repressor protein causing it to change shape so it can no longer bind to the operator so RNA polymerase can bind to the promoter and the three structural genes are transcribed and enzymes synthesised

Question 18

Describe what apoptosis is and the process of it

Answer 18

• The natural and controlled death of body cells and living tissue
• Enzymes break down the cytoskeleton
• Cytoplasm becomes dense with tightly packed organelles
• Plasma membrane changes and small protrusions called blebs form
• Chromatin condenses and nuclear membrane breaks down and DNA breaks into fragments
• Cell breaks into vesicles. Phagocytes infect and destroy these vesicles to prevent damage to other cells

Question 19

Describe what necrosis is and the process of it

Answer 19

• Un-programmed death of cells and living tissue (usually due to infection, injury or failed blood supply – typically involving the concurrent death of groups of adjacent cells
• Small blebs form and the nucleus changes structure
• The blebs fuse and become larger
• The cell membrane ruptures and releases the cells content

Question 20

Describe the similarities and differences between necrosis and apoptosis

Answer 20

• Both form small blebs
• In necrosis, the nucleus changes shape whereas in apoptosis the nucleus breaks apart and DNA breaks up
• In necrosis, there are no organelles in the blebs whereas there are organelles in the blebs in apoptosis
• In necrosis the cell membrane ruptures, releasing the cell content and making the organelles not functional whereas in apoptosis the cell breaks into apoptotic bodies with functioning organelles