Inheritance

Question 1

why should you double the DNA

Answer 1

ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA

Question 2

semi conservative replication

Answer 2

new DNA molecule has conserved half of the original DNA and then used this to create a new strand

The DNA is copied
In each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell

Question 3

importance of retaining one original DNA strand

Answer 3

ensures there is genetic continuity between generations of cells
This is important because cells in our body are replaced regularly and therefore we need the new cells to be able to do the same role as the old ones

Question 4

what happens after the enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands

Answer 4

Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base pairing
The new nucleotides are then joined together by the enzyme DNA polymerase
The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule

Question 5

how does the enzyme DNA polymerase synthesis new DNA strand

Answer 5

catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands

the two extra phosphates and uses the energy released to create the phosphodiester bonds

Question 6

how does the enzyme DNA polymerase synthesis new DNA strand

Answer 6

catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands

Question 7

what forms between complementary base pairs of template and the new DNA strands

Answer 7

Hydrogen bonds

Question 8

what direction is the new strand made and from what direction is DNA unzipped

Answer 8

DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)

As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork

Question 9

what is the leading strand

Answer 9

This template strand that the DNA polymerase attaches to

Question 10

when is the lagging strand

Answer 10

The other template strand created during DNA replication

Question 11

what does it mean if the DNA polymerase moves away from replication fork on lagging strand

Answer 11

DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments (called Okazaki fragments)

Question 12

what is needed to join lagging strands together

Answer 12

DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand

Question 13

what does DNA ligase do

Answer 13

catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

Question 14

what is a gene mutation

Answer 14

change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

Question 15

why do mutations not alter polypeptide

Answer 15

the genetic code is degenerate

Question 16

what is insertion of nucleotides

Answer 16

A mutation that occurs when a nucleotide is randomly inserted into the DNA sequence

Question 17

does insertion mutations have a knock on effect

Answer 17

An insertion mutation also has a knock-on effect on other base triplets by changing the triplets further on in the DNA sequence

This may dramatically change the amino acid sequence produced from this gene and therefore the ability of the polypeptide to function

Question 18

deletion of nucleotide

Answer 18

A mutation that occurs when a nucleotide is randomly deleted from the DNA sequence

changes the triplet in which the deletion has occurred, and also changes every group of three bases further on in the DNA sequence

Question 19

substitution of nucleotides

Answer 19

a base in the DNA sequence is randomly swapped for a different base

Question 20

what is a silent/missense/nonsense mutation

Answer 20

silent: mutation doesn’t alter the amino acid sequence

missense: alters a single amino acid

nonsense: creates a premature stop codon

Question 21

duplication mutation

Answer 21

production of one or more copies of a gene or a region of a chromosome

the second copy can undergo mutations which enable it to develop new functions

Question 22

inversion mutation

Answer 22

crossing-over in meiosis

The DNA of a single gene is cut in two places
The cut portion is inverted 180° then rejoined to the same place within the gene
The result is a large section of the gene that is ‘backwards’ and therefore multiple amino acids are affected

Question 23

what is a chromosome

Answer 23

long DNA molecule which contains genes that code for several different proteins

Question 24

what is a homologous pair of chromosomes

Answer 24

a pair of chromosomes that match in size and shape, and that contain the same genes at the same loci

Question 25

genotype

Answer 25

alleles of a gene possessed by that individual

Question 26

uses of genetic screening

Answer 26

Identifying individuals who are carrying an allele at a gene locus for a particular disorder

testing a foetus before birth; this is prenatal testing

The screening of embryos prior to implantation during fertility treatment

Question 27

what is IVF

Answer 27

fertilisation is carried out in the lab; embryos produced in this way can be implanted into the uterus where they develop into a foetus

Question 28

benefits of PGD

Answer 28

The sample of DNA to be analysed can be obtained by taking cell samples from embryos produced during IVF

Reduces the chances of having a baby with a genetic disorder
It avoids abortion as it is carried out before implantation of the embryo

Question 29

how is the DNA in prenatal testing obtained

Answer 29

chorionic villus sampling

amniocentesis

Question 30

explain chorionic villus sampling

Answer 30

This involves removing and testing a small sample of cells from the placenta using a fine needle
The cells contain foetal DNA which can be analysed for genetic disorders, allowing parents to make informed decisions about the pregnancy and foetus
It is carried out at around 11-14 weeks of pregnancy
The process has a 1-2 % risk of miscarriage

Question 31

explain amniocentesis

Answer 31

involves removing and testing a small sample of cells from amniotic fluid using a fine needle
The amniotic fluid is the fluid that surrounds the foetus within the uterus
The fluid contains foetal cells which contain DNA to be analysed
It is carried out at around 15-20 weeks of pregnancy
The procedure has a 1% risk of miscarriage

Question 32

why does a person with cystic fibrosis have such symptoms

Answer 32

mucus {thicker / stickier} than normal

(pancreatic) enzymes cannot enter intestine because pancreatic duct blocked with mucus
(high energy diet required because) digestion is less efficient
pancreatic enzymes trapped behind mucus damage pancreatic cells such as those that produce insulin
cysts form in pancreas

sperm cannot leave the testes because sperm duct blocked with mucus
* sperm duct absent therefore sperm cannot pass through

gene mutation
(causing a) non-functioning CFTR protein channel chloride ions cannot move out of epithelial cells
accumulation of sodium and chloride ions in the cells {causing water to move out of mucus by osmosis

Question 33

why does cystic fibrosis affect rate of oxygen uptake

Answer 33

{ thick / sticky / viscous } mucus (1)
* (accumulation of mucus) which cannot be moved by cilia (1)
* restricting air flow through { bronchioles / bronchi } (1)
* {increases diffusion distance / reduces surface area for gas exchange } in the alveoli (1)

Question 34

why should children be given dietary supplements and digestive enzymes

Answer 34

cystic fibrosis causes the production of {thicker / stickier} mucus (1)
which blocks the pancreatic duct
prevents} enzymes digesting {carbohydrates / lipids / proteins} (in intestines) (1)
(resulting in) reduced {absorption of products of digestion into the {blood(1)
* linkage of reduced vitamins to slower growth rate (1)
* {dietary supplements / digestive enzymes}
would increase growth rate (1)

Question 35

why not have chronic villus testing

Answer 35

test result may be inaccurate (1)
(increased) risk of miscarriage (due to the procedure) (1)
false positive may lead to termination of healthy fetus (1)
{ prenatal testing / abortion} against values or beliefs of the parents (1)

Question 36

What is a inherited recessive conditions

Answer 36

faulty allele expressed in homozygous recessive condition

Question 37

why offspring have palomino coat colour

Answer 37

(palomino / offspring) is heterozygous (1)
*
*
therefore { incomplete dominance / co-dominance } occurs (1)
because both alleles are expressed (1)

Question 38

one ethical issue with use of prenatal genetic screening

Answer 38

either
* it may result in a choice of an abortion (1)
* it is unethical to cause the death of a foetus (1)

(risk of) {incorrect result / false positive / false negative }
* healthy foetus could be aborted / parents not prepared for child with {genetic disease / achondroplasia} (1)

Question 39

why do offspring have palomino coat colour

Answer 39

because it is heterozygous so incomplete dominance occurs so both alleles expressed

Question 40

what is incomplete dominance

Answer 40

Both alleles influence the phenotype in a heterozygous individual