Genetic Screening Of Breast Cancer Essay Research

Genetic Screening Of Breast Cancer Essay, Research Paper

Hereditary breast cancer is a disease caused by mutations on breast cancer suppresser genes(ACCV Pg.17). Mutations allow normal cells to divide abnormally(ACCV Pg.13). Resulting cells divide faster as they do not specialise and form useless lumps of cells called malignant tumours(ACCV Pg.13).

Genetic Screening is the process where Deoxyribonucleic Acid (DNA) fragments are analysed for a specific gene. The purpose is to identify individuals carrying disease causing genes so they can change their life style and also help invent a cure(ACCV Pg.20). This is done by amplifying DNA withdrawn from an individual, then specific gene mutations are targeted using the Electrophoresis process.

The two genes, BRCA1 and BRCA2 isolated in 1994 and 1995 respectively are breast cancer suppresser genes(Internet 1). BRCA1 is located on chromosome 17q21 and BRCA2 on 13q(Internet 2). A person that possesses certain mutations to these genes has an increased risk of up to 80-90% in developing breast cancer(Internet 3).

The cost of genetic screening ranges among several hundred to several thousand dollars depending on the tests performed and can take several weeks to many months from the initial blood sample(Internet 4).

Public acceptance of genetic screening for severe disease causing genes in early childhood is high(New Scientist Pg. 14). Many people argue for less debilitating diseases that discrimination will occur against individuals carrying those genes(New Scientist Pg. 14).

In human cells there are 22 pairs of autosomal chromosomes and two sex chromosomes. These chromosomes contain information for protein synthesis. DNA stores this information by a sequence of nucleotides. There are four different nucleotides that construct DNA. They all contain a 5 ring carbon sugar (Deoxyribose), a phosphate molecule and one of four nitrogenous bases. The base names are Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Adenine is complementary to Thymine and Guanine to Cytosine. The arrangements of a series of nucleotides are genes. Hereditary Breast Cancer is an autosomal dominant disease(Internet 3), meaning only one parent needs to carry the trait expression in the parents offspring. The disease is cause by mutations found on the BRCA1 or 2 tumour suppresser genes(Internet 3).

BRCA1 has 24 exons distributed over a genomic region of 81 kilobases long and located on chromosome17q21(Internet 3)

Exon 11 being the largest that codes for 61% of a protein, 1863 amino acids and 5592 nucleotides long(ACCV Pg. 17). The irrelevant information known as introns found on BRCA1 range in size from 403 base pairs to 9.2 kilobases(Internet 3). Over 100 disease-associated mutations have be identified to this gene(Internet 3) 21 of these found in exon 11(ACCV Pg. 17). These mutations code for a stop signal causing protein truncation(ACCV Pg. 17).

BRCA2 has mutations that function the same as BRCA1(ACCV Pg. 18). BRCA2 has been linked to hereditary breast cancer and increases the risk for male breast cancer. (ACCV Pg. 18). BRCA2 is located on chromosome 13q12(Internet 2). Little additional detail about this gene is available. Testing for BRCA2 is not widely available except within the research laboratory.

There are two distinctive stages in protein synthesis of BRCA1, transcription and translation. Transcription is the synthesis of messenger Ribonucleic Acid (mRNA). The enzyme RNA polymerase initiates transcription by separation of DNA strands. RNA nucleotides then bind to their complementary DNA nucleotides of the BRCA1 gene to form a mRNA strand. The mRNA is different to the DNA strand of the BRCA1 gene as Uracil (U) replaces Thymine and is complementary to Adenine.

The resulting mRNA strand detaches from the BRCA1 gene before the DNA Ligase enzyme joins the DNA strands together. Splicing of the mRNA occurs to remove introns(Raven 440). The mRNA now only contains exons, that are primary transcripts of the gene. The mRNA strands leave the nucleus through nuclear pores to undergo Translation the second stage of protein synthesis.

Translation occurs at the ribosome found in the cytoplasm, where production of the tomour suppresser protein from mRNA occurs. A ribosomal RNA molecule with in the ribosome binds to the “start” sequence of the mRNA strand. The ribosome then moves the mRNA strand through 3 nucleotides adding an amino acid. This process continues until the ribosome encounters a “stop” signal at this point it disengages from the mRNA and releases the completed suppresser protein.

Genetic screening can allow testing DNA to determine if an individual carries mutated forms of the BRCA1 gene.

DNA collection is the first stage to screen for the BRCA1gene. White blood cells withdrawn from a blood sample contain the needed DNA. The DNA needs amplification so that a large volume of DNA is available for analysis. Polymerase Chain Reaction (PCR) is a process that amplifies DNA(ACCV Pg. 20). Endonuclease restriction enzyme shortens the DNA for amplification. This enzyme recognises the BRCA1 gene and cuts it from the DNA fragment(ACCV Pg. 20).

PCR is sensitive and requires a gene that is between known two sequences such as BRCA1(ACCV Pg. 21). Short complementary sequences to the BRCA1 gene, oligonucleotides, known as a primer is added to th