2.16 - 2.18 cystic fibrosis & genetic screening

Question 1

cystic fibrosis

Answer 1

a serious genetic disease caused by a recessive allele which affects the production of mucus by epithelial cells. it results from one of a number of possible mutations
the chloride transport system of the exocrine glands don’t function properly, leading to production of a thick sticky mucus

Question 2

exocrine glands

Answer 2

glands which produce substances and secrete them to where they are needed through a small tube called a duct

Question 3

villi

Answer 3

finger-like projections of the lining of the duodenum and small intestine which increase the surface area for the absorption of digested food

Question 4

duodenum

Answer 4

the first part of the gut after the stomach

Question 5

pancreatic duct

Answer 5

the duct from the pancreas which carries digestive enzymes made in the pancreas into the duodenum

Question 6

insulin

Answer 6

a hormone made in the pancreas involved in the regulation of blood sugar levels

Question 7

genetic screening

Answer 7

when whole populations are tested for a genetic disease

Question 8

phenylketonuria (PKU)

Answer 8

a recessive genetic disorder where those affected lack the enzyme needed to digest the amino acid phenylalanine; the amino acid builds up in the blood and causes severe brain damage

Question 9

prenatal screening

Answer 9

screening of an embryo or fetus before birth

Question 10

amniocentesis

Answer 10

a type of prenatal screening which involves removing a sample of amniotic fluid at around 16 weeks of pregnancy, culturing the fetal cells found and analysing them for genetic diseases by karyotyping

Question 11

chorionic villus sampling (CVS)

Answer 11

a type of prenatal screening where a small sample of embryonic tissue is taken from the developing placenta and the cells are tested for genetic diseases by karyotyping

Question 12

preimplantation genetic diagnosis

Answer 12

testing the cells of an embryo produced by IVF (in vitro fertilisation) to check for genetic diseases before it is implanted into the uterus of the mother

Question 13

genetic counselling

Answer 13

help people to understand and come to terms with the situation of carrying a faulty allele that can cause a genetic disease, assess the statistical risk of producing an affected child, and help couples recognise their options

Question 14

what are the disadvantages of amniocentesis? (3)

Answer 14

1. it can only be carried out relatively late in the pregnancy, making it very difficult for the parents if termination of the pregnancy is necessary
2. the results are not available until 2-3 weeks after the test
3. it carries a risk of spontaneous abortions after the procedure, regardless of the genetic status of the fetus

Question 15

what are the disadvantages of chorionic villus sampling? (2)

Answer 15

1. there is a risk that the embryo may spontaneously abort after the tissue sample is taken, though the risk of miscarriage at this stage of pregnancy is high anyway
2. all paternal x chromosomes are inactivated in fetal placental cells so any problems in the genes on that chromosome cannot be detected by this technique

Question 16

what is the cause of cystic fibrosis?

Answer 16

when an individual inherits 2 copies of the recessive faulty allele that codes for the CFTR channel protein which transports chloride ions across the cell membrane, they cannot synthesise effective CFTR proteins.

Question 17

CFTR channel protein

Answer 17

it is an enormous protein containing 1480 amino acid. it lines the channels through which chloride ions leave the epithelial cells and move into the fluid outside the cells in the lungs

Question 18

how does the absent/ dysfunctional CFTR channel protein affects the epithelial cells?

Answer 18

the chloride transport systems of the exocrine glands don’t function properly to transport chloride ions out of epithelial cells because of faulty CFTR proteins.
the chloride ions build up inside epithelial cells instead of moving out through the CFTR channels.
water moves into the cells by osmosis, making the mucus thick and sticky. cilia are trapped in thick mucus so they cannot breathe effectively.

Question 19

what are some problems of the respiratory system caused by cystic fibrosis? (10)

Answer 19

1. thick, sticky mucus builds up in the tiny airway of the lungs and reduces the flow of air into the alveoli & bronchioles
2. obstructs smaller bronchioles completely
3. smaller gas concentration gradient between the air and the blood in the lungs reduces gas exchange
4. reduces the surface area for gas exchange
5. severe coughing as the body tries to expel the mucus
6. breathless and short of oxygen
7. tiredness & lack of energy
8. cilia cannot move the thick and sticky mucus out of the system
9. bacteria and other pathogens are trapped in the mucus and builds up in the lungs, giving them the ideal conditions to grow, eventually causing lung infection
10. the dehydrated surfaces of the cells prevents antibodies from functioning effectively in the thickened mucus (however it can be rehydrated which can restore the function of the antibodies)

Question 20

how does cystic fibrosis affect the digestive system (2)

Answer 20

1) thick and sticky mucus produced by the pancreatic cells line the pancreatic duct in the same way that the airways of the lungs produce it because of the faulty CFTR protein
the mucus blocks the pancreatic duct
pancreatic enzymes that break down carbs, proteins and fats do not reach the duodenum
2) thick and sticky mucus acts as a barrier between food and the intestinal linings and clogs up the villi, reducing the surface area for absorption

Question 21

what are some problems of the digestive system caused by cystic fibrosis? (4)

Answer 21

1. poor food digestion
2. trapped enzymes digest and damage pancreatic cells
3. affect the pancreatic cells that make insulin so the patient may develop diabetes
4. severe risks of malnutrition and struggle to maintain body mass

Question 22

how does cystic fibrosis affect the productive system of women?

Answer 22

the mucus in the system normally changes through the menstrual cycle. when a woman is fertile, the mucus becomes thinner to help sperm get through the cervix and along the oviducts.
women with CF usually product fertile eggs but the thick mucus can block the cervix and oviducts so sperm cannot reach them

Question 23

how does cystic fibrosis affect the reproductive system of men?

Answer 23

men with CF are often infertile. they lack the sperm duct that carries the sperm out from the testes into the semen. even if the sperm duct is present, it may be partly or completely blocked by the thick and sticky mucus so less of no sperm can leave the testes

Question 24

how does cystic fibrosis affect the sweat glands?

Answer 24

it makes the sweat glands produce more concentrated and salty sweat because the faulty CFTR proteins cannot move the chloride ions into the cells.

• the loss of sodium and chloride ions can cause health problems involving the nervous system and the heart

Question 25

how do the sweat glands of healthy people work?

Answer 25

sweat of healthy people is mainly salty water. the sweat passes along the duct of a sweat gland and salt is re absorbed because of CFTR protein moving chloride ions into the cells. this prevents individuals from losing to much salt in sweat.

Question 26

what is the process of preimplantation genetic diagnosis?

Answer 26

1. the egg and sperm are fertilised outside the body by in vitro fertilisation 2. after a few cell divisions, a single cell is removed from each embryo 3. the genetic make-up of the cell is checked 4. only embryos without faulty alleles are implanted in the mother’s uterus to grow

Question 27

what are the benefits of preimplantation genetic diagnosis?

Answer 27

1. the process does not harm the embryo in any way 2. the chances of having a baby with a genetic disorder is reduced 3. abortion can be avoided as it is carried out before the implantation of the embryo

Question 28

what are the advantages of chorionic villus sampling? (4)

Answer 28

1. much larger sample of feral tissue is available for examination so cell culture is not needed 2. it can be tested for a wide range of abnormalities 3. the test can be carried out earlier in pregnancy so decisions of whether to continue the pregnancy or not can be made earlier 4. results are available sooner than amniocentesis