Biology Topic 2 Part 3- Cystic Fibrosis

What is a gene mutation

a change in the base sequence of a gene and can happen spontaneously during DNA replication

Why do most mutations not alter the polypeptide chain

because the genetic code is degenerate

What is meant by degenerate

Most amino acids are coded for by more than one triplet

Different causes of mutation

Insertion

Deletion

Substitution

Duplication

Inversion

Insertion

A mutation involving the addition of one or more nucleotide pairs to a gene. This can change every triplet in a DNA sequence, so could change the ability of the polypeptide function

Deletion

A change to a chromosome in which a fragment of the chromosome is removed. Same as an insertion, just one is removed

Substitution

A mutation in which a nucleotide or a codon in DNA is replaced with a different nucleotide. Only has an affect on one triplet, so may not alter polypeptide function

Types of substitution mutation

1. Silent- The mutation does not alter the amino acid sequence of the polypeptide; this is due to the degenerate nature of the genetic code

2. Missense- The mutation alters a single amino acid in the polypeptide chain

3. Nonsense- The mutation creates a premature stop codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function

Duplication

Duplication is a type of mutation that involves the production of one or more copies of a gene or a region of a chromosome. Important for evolution. More common in plants

Inversion

Inversion mutations usually occur during 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

What is cystic fibrosis

Cystic fibrosis is a genetic disorder of cell membranes caused by a recessive allele of the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene located on chromosome 7

What is the role of the CFTR gene

Codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive juices

What happens if there is a mutation in the CFTR gene

A mutation in the CFTR gene leads to production of non-functional chloride channels. This reduces the movement of water by osmosis into the secretions. The result is that the body produces large amounts of thick, sticky mucus in the air passages, the digestive tract and the reproductive system

How does cystic fibrosis affect the respiratory system

Mucus in the respiratory system keeps the lungs healthy. It prevents infection by trapping microorganisms. This mucus is moved out of the respiratory tract by cilia. In people with cystic fibrosis, due to the faulty chloride ion channels, the cilia are unable to move as the mucus is so thick and sticky. This means microorganisms are not efficiently removed from the lungs and lung infections occur more frequently. Mucus builds up in the lungs and can block airways which limits gas exchange. The surface area for gas exchange is reduced which can cause breathing difficulties. Physiotherapy can support people with cystic fibrosis to loosen the mucus in the airways and improve gas exchange

How does cystic fibrosis affect the digestive system

Thick mucus in the digestive system can cause issues because the tube to the pancreas can become blocked, preventing digestive enzymes from entering the small intestine, reducing digestion of food and therefore key nutrients may not be made available for absorption. The mucus can cause cysts to grow in the pancreas which inhibit the production of enzymes, further reducing digestion of key nutrients. The lining of the intestines is also coated in thick mucus, inhibiting the absorption of nutrients into the blood

How can cystic fibrosis affect the reproductive system

Mucus is normally secreted in the reproductive system to prevent infection and regulate the progress of sperm through the reproductive tract after sexual intercourse. The mucus in people with cystic fibrosis can cause issues in both men and womenIn men the tubes of the testes can become blocked, preventing sperm from reaching the penis. In women thickened cervical mucus can prevent sperm reaching the oviduct to fertilise an egg

What is incomplete dominance

This occurs when the trait from a dominant allele is not completely expressed over the trait produced by the recessive allele. When present the recessive allele 'dilutes' the dominant allele. Both alleles influence the phenotype in a heterozygous individual, giving rise to a third phenotype

What is monohybrid inheritance

Inheritance of a characteristic controlled by a single gene

What is genetic testing

The use of methods to determine if someone has a genetic disorder, will develop one, or is a carrier

How can genetic screening be used

1. To confirm a diagnosis- a negative result should be treated with caution, as there are many different mutations of the CFTR gene that can lead to cystic fibrosis and we cannot test for every single one

2. To identify carriers- A sample of blood or cells taken from inside the mouth to detect abnormal alleles.

3. For testing embryos- 3 types of embryonic testing

What are the three types of embryonic genetic screening

Amniocentesis

Chorionic Villus Sampling (CVS)

Non-invasive Prenatal Diagnosis (NIPD)

Amniocentisis

A needle is inserted into the amniotic fluid to collect fetal cells that have fallen off of the placenta and fetus. Happens at around 15-17 weeks of pregnancy and there is a 1% chance of miscarriage

Chorionic villus sampling (CVS)

A small sample of placental tissue is removed, either through the wall of the abdomen or through the vagina. Carried out between 8 and 12 weeks, as you don't need to wait for amniotic fluid to develop. Higher risk of miscarriage, around 1-2%

Non-invasive prenatal diagnosis (NIPD)