AS Biology Chapter 2.4 - Genetic Diseases

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Chapter 2.4 ­ In depth study of a genetic disease.
Cystic fibrosis is caused when the chlorine transport system of the exocrine glands, including the
mucus-secreting glands of the airways of the lungs, the gut and the reproductive system and the
sweat glands, don't function properly.
Cystic fibrosis transmembrane regulatory (CTFR) is a large protein. The gene that codes for it is also
large, and is produced on chromosome 7. A mutation in any part of the gene can affect the CFTR
protein, and so cause cystic fibrosis.
The most common mutation is known as DF508 and is found in 75% of people with cystic fibrosis.
People who inherit two copies of the faulty allele lack effective CFTR proteins. This means that the
chloride ions build up in the cells instead of moving out through the channels. So the water doesn't
move out of the cells to dilute the mucus on the surface of the membranes. So water moves into the
cells by osmosis from the fluid surrounding the cells, making the mucus even more thick and sticky.
Cystic fibrosis is carried by a recessive allele.
Cystic Fibrosis on the Respiratory System
The thick, sticky mucus builds up in the tiny airways of the lungs and reduces the flow of air
into the alveoli.
The mucus blocks the smaller bronchioles and reduces the surface area available in the lungs
for gaseous exchange.
So the affected individuals have severe coughing fits as their body attempts to get rid of the
The mucus is so thick and sticky that the cilia lining the airways cannot move it along and out
of the system.
Bacteria and pathogens which are breathed in are trapped in the mucus and can't be moved
out of the respiratory system.
Cystic Fibrosis on the Digestive System
The digestive system makes enzymes which break down large complex food molecules into
smaller molecules. These are then absorbed into the blood through the lining of the small
intestine which is covered in villi, which increases the surface area for absorption to take
Enzymes from the pancreas are important in the breakdown of carbohydrates, proteins and
fats in the top part of the small intestine (duodenum). The enzymes are passed from the
pancreas into your duodenum along the pancreatic duct.
Mucus is produced by the cells lining this tube (pancreatic duct). A faulty CFTR protein
means the mucus is thick and sticky. It blocks the pancreatic duct, so that the enzymes don't
reach the duodenum.
If the digestive enzymes don't reach the gut, you can't digest your food properly, so you
don't get enough nutrients from the food.
The digestive enzymes trapped in the pancreas may start to digest and damage the cells of
the pancreas.

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Mucus is secreted throughout the gut to protect it from damage by the digestive enzymes.
But if the mucus is thick and sticky, it forms a barrier between the contents of the gut and
the lining of the small intestine, and clogs up the villi reducing the surface area for
absorption. This can lead to malnutrition.
Cystic Fibrosis on the Reproductive System
When a woman is fertile, the mucus becomes thinner to help the sperm get through the
cervix and along the oviducts.…read more

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Cystic Fibrosis blocks the airways and the surface is coated with thick, sticky mucus preventing the
new DNA in the vectors from reaching the epithelial cells.
Prenatal screening ­ Each pregnancy is screened. To see if the fetus has CF you carry out tests on
some fetal cells. These are obtained by:-
Amniocentesis ­ 20cm³ of amniotic fluid surrounding the fetus is removed around the 16th
week of pregnancy. After the cells have been cultured for 2-3 weeks you get the results.…read more


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