Molecular Genetics Part 2
Explain the process of protein synthesis
MRNA acts as the intermediate between DNA and ribosomes, is translated into proteins by ribosomes, RNA version of the gene encoded by DNA, during transcription DNA code is chemically re-written as an RNA code, it happens in the nucleus, there are three parts, initiation elongation and termination, Initiation – RNA polymerase binds to the DNA and unwinds it near beginning of gene, this binding occurs at promoter, key element in promoter is the Tata box which is section of DNA high in thymine and adenine recognized by RNA polymerase, elongation – once the RNA polymerase binds to promoter and opens ip the helix it starts to build the single stranded RNA, made in 5’ to 3’ direction, the opposite strand of the DNA is the coding strand, RNA polymerase unwinds DNA as it moves along, when its done it unwinds as the enzyme passes, termination – transcription stops when RNA polymerase recognizes a termination sequence
Post transcriptional modifications
At the end of transcription the newly formed RNA is vulnerable to conditions outside the cell nucleus, it must undergo additional modifications before exiting the cell, the mRNA contains useful code and junk code, the jink code is called introns and cut out by enzymes called spliceosomes, the useful code called exons is spliced together, this will be the final version used to make protein, capping is when the beginning of the pre mRNA transcript where 5’ cap of seven Gs is added, the cap functions as the initial attachment site for mRNAs to ribosomes to allow for translation,tailing is when the addition of a chain of 5- to 250 adenine nucleotides to the 3’ end by poly A polymerase, this tail enables mRNA to be translated efficiently and protects it from attack by RNA digesting enzymes in the cytosol
Translation
Translation uses the DNA message that was copied out into mRNA to produce a polypeptide or protein, this is the second part of the central dogma, relies on genetic code, tRNA – single stranded but fold upon into a clover like shape, able to bind to amino acids at one end and to mRNA at the other, mRNA binding end has an anticodon, each anticodon codes for different amino acids, amino acids bind at the 3’ of tRNA, the anticodon binds to complementary codon sequence, Ribosome is the site of translation, can be free in the cytoplasm, two subunits large and small binds tRNA at one of three sites exit, peptidyl and aminoacyl, the first tRNA binds to the P site, the second tRNA binds to the A site, this brings the amino acids on each tRNA close enough to form a peptide bond, as the ribosome shifts down the first tRNA is bumped into the E site and released, there are 20 different amino acids, initiaition – mRNA bidns to the ribosome, tRNAs carrying amino acids arrive and peptide bond forms between amino acids, elongation – the ribosome shifts moving the tRNA down the p, a and e line and opeing the A site for a new tRNA to bind, many ribosomes can bind to the same mRNA and translate it simutaneously amplifying the amount of protein made, termination – three mRNA codons that signal the end of a protein, these are called stop codons (UAA, UAG and UGA, when it reaches a stop codon the ribosome releases the mRNA and translation ends
Describe the regulation of gene expression
Two examples of regulation are concetration of molecules lactose and tryptophan both examples of negative feedback control, operon – a stretch of DNA that contains a set of one or more genes involved in metabolic pathway, promoter is a sequence of DNA that binds RNA polymerase its where transcription begins, operator is sequens that controls transcription, repressor is a protein that regulates the production of genes in the operon, inducer is a molecule that stops the action of the repressor, co repressor is a signal molecule that binds to a regulatory protein to reduce the expression of operon genes,Lac operon – there are 3 genes that code for proteins involved in the breakdown of lactsose in E coli, no lactose present, a repressor binds to the operator region of the lac operon, makes it impossible for RNA polymerase to bind to promoter, lactose present, some lactose will bind to the repressor and cause it to detach from the operator site, polymerase can now attach to lac operson and transcribe genes to metabolize lactose, lactose acts as a inducer, trp operon – when trp is absent the trp repressor is inactive in binding to the operator and transcription proceeds, typtophan is present the amino acid binds to and activates the trp repressor, the repressor binds to the operator and blocks transcription
Discuss the effects of mutations
Genetic mutations are changes in the DNA sequence caused by various mechanisms, can be small scale or large scale, can be spontaneous or induced, can be positive or neutral, small scale mutations are of an individual base pair and of small groups of base pairs, missense mutation – a change in the base sequence of DNA that alters the codon leading to a different amino acid being produced, nonsense mutation – a change in the DNA/RNA which causes a stop codon to replace a codon specifying an amino acid, silent mutation – a mutation that has no effect on the operation of the cell the same amino acids are still coded, frameshift mutation occurs when one or more nucleotides are inserted or deleted from a DNA sequence causing the reading frame of codons to shift in one direction or another, amplication – occurs when a gene or group of genes is copied to multiple regions of a chromosome, leads to large number of copies of the gene or gene group, Large scale deletions – entire coding regions of DNA are removed, unless multiple copies are available, large loss of genetic material negatively effects the cell, Translocation – movement of entire genes or sequences of DNA from one chromosome to another, if translocated adjacent can result in an entirely new gene, Inversion – occurs when a portion of DNA containing one or more genes reverses its direction in the genome, if break occurs in the middle of coding sequence the gene can be compromised
State causes of mutations
Spontaneous mutations are caused by errors in DNA replication, induced mutations are caused by an environmental. Agent known as a mutagen. Two common mutagens are certain chemicals and radiation
Describe some positive and negative effects of mutations
The majority of mutations are neutral, in eukaryotes 90% of DNA is non coding or junk, in prokaryotes a mutation is more likely to be harmful, a mutation is only beneficial in a given time and situation, ectrodactyly is when people have a cleft where middle finger or toe should be it is caused by several factors like deletions, translocation, and inversions in chromosome 7, some people have a mutation that disables their CCR5 protein and HIV uses that protein to enter cells,if a person lacks this HIV cant enter the cells, they are resistant rather than immune
Explain genetic manipulation
Genetic engineering is the intentional production of new genes and alteration of genomes by the substitution or introduction of new genetic material, A plasmid is a small circular DNA molecule found in bacteria, recombinant DNA is a DNA strand that is created using DNA pieces from 2 or more sources, restriction enzymes recognize a specific DNA sequence of bases and cut the DNA at the recognition site, most recognition sites are 4- 8 bases long and are palindromes (something that reads the same backwards and forwards), gel electrophoresis is a way of analyzing a DNA sample, DNA is copied by PCR then cut into fragments by restriction enzymes and embedded into agarose gel, once in gel it has electric charge and lighter objects move further, fragments are hybridized with a DNA probe, probes bind to complementary sequences and the different DNA samples can be analyzed and compared