Week 3 - Genetics

Question 1

What are non-sex chromosomes called

Answer 1

Autosomes

Question 2

What two things is DNA associated with?

Answer 2

Histones - small, positively charged proteins which have two of each H2A, H2B, H3 and H4 (octamer) - highly conserved in evolution

Chromatin - Non-histome chromosomal protein

Question 3

Explain how chromosome structure is dynamic

Answer 3

More condensed during mitosis than other phases of the cell cycle - less condense to allow gene expression, repair and replication

Question 4

How can chromatin be made more accessible?

Answer 4

Histone tails can be reversibly chemically modified

Question 5

What is a nucleosome?

Answer 5

One histone core and the DNA wrapped around it

Question 6

What is the centromere?

Answer 6

Highly repetitive DNA sequence which can stretch over megabases (10^6) - constriction point of a pair of chromatids - bound by large protein complexes called kinetochores

Question 7

What are telomeres?

Answer 7

Long, repetitive DNA sequence (TTAGGG) which forms a loop. Protects the end of the chromosome from natural cellular exonucleases, maintaining chromosome integrity

Question 8

What is a nucleotide?

Answer 8

A 5-carbon sugar (2-deoxyribose), phosphate and a base (nitrogen containing ring)

T as in THREE

Question 9

What is a nucleoside?

Answer 9

Just a base and a sugar

Adenosine, guanosine

Cytidine, thymidine

Question 10

What are pyrimidines?

Answer 10

Single ring

cYtosine and thYmine

Question 11

What are purines?

Answer 11

Double ring

adenine and guanine - neither have a Y

Question 12

What is thymine replaced with in RNA?

Answer 12

Uracil

Question 13

How many hydrogen bonds between cytosine and guanine?

Answer 13

3

Question 14

How many hydrogen bonds between thymine and adenine?

Answer 14

2

Question 15

Why are DNA strands called antiparallel?

Answer 15

They run in opposite directions

Question 16

What is the 5’ end?

Answer 16

The start of the chain, has a free 5’ phosphate

Question 17

What is the 3’ end?

Answer 17

The end of the chain, has a free 3’ OH

Question 18

Which end does DNA polymerase add nucleotides to?

Answer 18

3’ 3 is lower than 5, needs to be added to

Question 19

Which stand is synthesized continuously?

Answer 19

Leading strand

Lagging strand is synthesized in sections called okazaki fragments - later fused by DNA ligase

Question 20

What is DNA polymerase known as?

Answer 20

A primase - as it makes a RNA primer first

Question 21

What helps unwind the double helix?

Answer 21

Helicase

Question 22

How many primers needed for each strand?

Answer 22

Leading requires one

Lagging has continuous requirement - later removed by nucleases

Question 23

How often does DNA polymerase make a mistake?

Answer 23

One in 10^7 bases

Overall accuracy is 10^9 from repair

Question 24

What can UV light cause?

Answer 24

Thymine dimers

Question 25

Which is DNA replication called semiconservative?

Answer 25

Half of the parent strand is kept

Question 26

What DNA sequences are likely to be evolutionarily constrained?

Answer 26

Sequences which are functionally important

Question 27

Describe the differences between DNA and RNA

Answer 27

``` DNA is long - RNA is short Double stranded - single stranded Simple double helix structure - complex 3D like a protein Chemically stable - less stable Deoxyribose - ribose Thymine - uracil ```

Question 28

What is a gene?

Answer 28

A region of DNA which controls a discrete hereditary characteristic, usually corresponding to a single protein or RNA

Question 29

How many cells does an adult human have

Answer 29

~10^13

Question 30

What is a promoter?

Answer 30

Sequences immediately before 5' end of gene that interact with RNA polymerase

Question 31

What is an enhancer?

Answer 31

Increases transcription from a nearby gene - can operate over considerable distances

Question 32

What are transcription factors?

Answer 32

Proteins which bind to specific DNA sequences within a promoter or enhancer to increase or decrease gene expression

Question 33

How can expression of a single gene be controlled?

Answer 33

Alternative promoters Alternative splicing Alternative polyadenylation

Question 34

What is the difference between exons and introns

Answer 34

EXons are EXpressed - Code for proteins Introns are intervening sequences

Question 35

What is polyadenylation?

Answer 35

A polyA tail is a string of adenylate residues added to the 3' end of an mRNA - not on tRNA or rRNA

Question 36

What are the untranslated regions?

Answer 36

5' upstream of translated region - may affect translational control 3' downstream of translated region - can determine stability of the mRNA

Question 37

Explain PCR

Answer 37

Used for genotyping to detect mutations Heated to 95 to cause 2 strands to separate Cooled to 55 to allow primers to anneal Heated to 72 to synthesise new DNA strands using thermophilic DNA polymerase

Question 38

Genotype and phenotype difference

Answer 38

Genotype is an individual's genome Phenotype is their set of characteristics

Question 39

Why is there redundancy in codons?

Answer 39

64 codons (4x4x4) and only 20 amino acids plus 3 stop codons

Question 40

Describe a ribosome

Answer 40

Made of rRNA and proteins Had large and small subunits A - aminoacyl tRNA site P - peptidyl tRNA site E - Exit site mRNA binding site

Question 41

Describe the cycle of translation

Answer 41

Recognises start codon (methionine) which is usually cleaved away mRNA codon determines which tRNA binds New peptide bond formed between amino acids in the P and A sites Large subunit translocates and shifts tRNAs into the E and P sites Small unit translocates and the ribosome is reset with vacant A site

Question 42

Describe crossing over

Answer 42

When homologous chromosomes line up, genetic information can cross over at points called chiasmata between non-sister chromatids

Question 43

Describe independent assortment

Answer 43

Maternal and paternal chromosomes are shuffled and dealt randomly at meiosis 1 2^N different gametes In humans - 2^23 = 8.4 million genetically distinct gametes

Question 44

Aneuploidy vs polyploidy

Answer 44

Aneuploidy is more or less of one chromosome Polyploidy is more than 2 sets of chromosomes

Question 45

What's the law of segregation

Answer 45

2 alleles for each trait segregate during gamete formation and then unite at random during fertilisation

Question 46

What is the law of independent assortment

Answer 46

Alleles of two different genes get sorted into gametes independently - the allele a gamete receives for one gene does not influence the allele received for another Genes closer together on a chromosome are more likely to be inherited as a unit

Question 47

What are some endogenous sources of mutation

Answer 47

Spontaneous damage, errors in chromosome segregation, recombination, DNA replication, DNA repair

Question 48

What does deamination lead to?

Answer 48

C --> T transition

Question 49

What does xeroderma pigmentosum lead to?

Answer 49

Unable to repair thymine dimers - extensive skin cancer

Question 50

How can genetic variation cause disease?

Answer 50

Loss of function - becomes useless Gain of function - can be harmful Too much gene product Too little gene product

Question 51

Name types of mutations

Answer 51

Point - aka substitution - likely resultant protein is still functional - can be synonymous (where the protein is the same) or non synonymous (where it's different) non synonymous can be: - nonsense - codes for a stop codon - truncated protein - missense - creates an amino acid with different chemical properties - splice site - changes order of Exons and introns Deletion - frameshift - less likely to make a functional protein Insertions - frameshift - less likely to make a functional protein

Question 52

Types of chromosomal mutations

Answer 52

Non-allelic homologous recombination where one sequence is deleted and another is duplicated

Question 53

Give an example of how modifying the environment can have an influence in outcome when there is a mutation

Answer 53

PKU metabolic disease Mutation in gene encoding phenylalanine hydroxylase Cannot convert phenylalanine to tyrosine which results in toxin buildup leading to intellectual disability Can be diagnosed with a heel prick test (Guthrie test) Infant can be put on a low phenylalanine diet

Question 54

How can histone tails be genetically modified?

Answer 54

Methylation Acetylation Phosphorylation