Lecture 3

Question 1

How are errors in DNA repaired during replication?

Answer 1

-can be repaired on spot, DNA polymerase III can go backwards and cut the incorrect bases (exonuclease) function the proteins of the replication complex excise the incorrect nucleotides -some people don’t have this ability exoderma (the huge growths on skin etc.)

Question 2

How are errors in DNA repaired after replication?

Answer 2

mismatch repair:- when misread in replication or paired wrongly -mismatch repair proteins excise the wrong sequence plus some adjacent nucleotides, DNA polymerase I adds the correct nucleotides and ligase repairs the nicks. -if environmental damage to the sequence- enzymes can repair it in the same way excision repair: damage response -a nucleotide in DNA is damaged -the excision repair proteins excise the damaged nucleotide and some adjacent ones -DNA polymerase I adds correct nucleotides by 5’- 3’replication of the short strand

Question 3

Describe PCR!

Answer 3

-add DNA sequence to be amplified, 4 deoxyribonucleotide triphosphates (carrying the bases), deoxyribose, enzyme and 2 primers complementary to 3’ends of both strands -heat up to 95 C to separate DNA strands (hydrogen bonds break) -cool down to 60-65 C then the primers bind need to know the end of the added sequence so you can design the primer(must be complementary to the end sequence)-need primer for the lagging strand -can amplify regions of up to 2000 bases (2kb) -so sensitive you can use primers with the whole human genome and amplify a specific sequence -sequence of DNA can be amplified millions of times after several cycles of PCR

Question 4

Why is the temperature in PCR an issue?

Answer 4

Normally enzymes in DNA replication denature at 37 C. So here an enzyme from a bacterium living in hot springs is added Taq polymerase (type of DNA polymerase III of bacterium Thermus aquaticus) this adds the bases to the template DNA

Question 5

Is chromosome number constant during organism’s lifetime?

Answer 5

-yes

Question 6

What is aneuploidy?

Answer 6

extra chromosomes but not a set! -Eg. trisomy 2N+1, monosomy 2N-1 -happens in meiosis, non-disjunction at Anaphase I or Anaphase II (or both)

Question 7

What is klinefelter syndrome?

Answer 7

XXY -all males sort of… -extra chromosome X (bad, some genes 3x-messes it up) -varying severity of retardation -many die before the age of 5 (usually heart failure) -the ones who survive, in their 40s- Alzheimer like disease (get amyloid deposits in brain as chromosome 21 is an amyloid precursor) -enlarged tongue -males: enlarged breasts, small penis, slow learner, infertile

Question 8

What is euploidy?

Answer 8

extra sets (adding or taking sets), polyploidy, triploidy 3N, tetraploid 4N -can tolerate more sex chromosomes but not autosomes -polyploidy(more than 2 sets) often in agriculture -triploid bananas (3N) tetraploid grapes (4N) – reproduce vegetatively -triploid: three of each type of chromosome, humans- 69 3N- odd numbers of polyploidy usually result in death

Question 9

What are the effects of polyploidy in some animals?

Answer 9

: reproduction is then parthenogenic (without fertilization) - lizards, salamanders - triploid oysters do not spawn (spawning females unpalatable)

Question 10

What is the effect of triploidy in humans?

Answer 10

human triploidy leads to death in-utero or soon after birth, not uncommon, very common cause of miscarriages (especially older mothers)

Question 11

What are the 5 ways of preventing human aneuploidy that are used in pregnancy?

Answer 11

-pre-implantation diagnosis -ultrasound -triple test of maternal blood -chorionic villus sampling -amniocentesis

Question 12

What is pre-implantation diagnosis (PID)?

Answer 12

when IVF treatment : take out a single totipotent cell from an embryo before implantation

Question 13

What is ultrasound?

Answer 13

8-10 weeks and second at 20 weeks that can pick up gross abnormalities and then further testing

Question 14

What is the triple test of maternal blood?

Answer 14

measures 3 compounds(eg. protein= indication of open spinal chord, skull) which give an indication of possible abnormalities (around the time of first ultrasound)

Question 15

What is chorionic villus sampling?

Answer 15

10- 12 weeks, chorion is a tissue that has the same genetic make up as the fetus, more invasive, guided by ultrasound

Question 16

What is amniocentesis?

Answer 16

later, withdrawing fluid, need 30ml, 14-16 weeks-then enough fluid, guided by ultrasound, if termination at this stage- have to get through labour, get DNA, karyotype

Question 17

What is a karyotype?

Answer 17

sorted chromosomes

Question 18

What is a karyogram?

Answer 18

a drawing of chromosomes with banding shown

Question 19

What is an autosome?

Answer 19

most chromosomes, -not involved in determining the sex of an organism

Question 20

What is a sex chromosome?

Answer 20

a chromsome involved in determining the sex of an organism

Question 21

What are alleles?

Answer 21

alternative forms of genes, arise as a result of mutation -after DNA replication the alleles on a gene are identical - homologous chromosomes have the same genes but may have different alleles

Question 22

What are the rules for naming alleles?

Answer 22

-usually letters of mutations= allele letters -humans, letters in capitals

Question 23

What are non-sister chromatids?

Answer 23

-when one from mother one from father, not held by centromere

Question 24

What is a wild type allele?

Answer 24

most frequently occurring allele in a population (not used in humans)

Question 25

What is a mutant type allele?

Answer 25

• the less frequently occurring allele

Question 26

What is the gene locus?

Answer 26

position of the gene on the chromosome

Question 27

What is a homologous chromosome?

Answer 27

-

Question 28

Describe what happens in a monohybrid cross with complete dominance (1 gene, 2 alleles).

Answer 28