Genetic Processes PDF

THE FLOW OF GENETIC INFORMATION Genetic Code
The sequence of bases that encodes a functional protein is called the gene
The relationship between the base sequence and the amino acid sequence in a particular protein is called the genetic code
A codon consists of 3 nucleotides http://ww...

THE FLOW OF GENETIC INFORMATION Genetic Code
The sequence of bases that encodes a functional protein is called the gene
The relationship between the base sequence and the amino acid sequence in a particular protein is called the genetic code
A codon consists of 3 nucleotides http://www.mun.ca/biology/scarr/MGA2-03-28.jpg Mutation  An error in the copying of a sequence of bases - Mutation  Mutation can occur at the DNA level but is expressed at the protein level  Organic compounds that induce mutations by reacting with DNA – Mutagens  Many changes that are caused by radiation and mutagens do not become mutations because the cells have a repair mechanism which can prevent mutations  Some mutagens can cause cancer – Carcinogens  Most carcinogens are mutagens, however most mutagens are not carcinogens   Not all mutations are harmful…some mutations increases the survival rate of species A harmful mutation results in an inborn genetic disease Consequences of Altering the Nucleotide Sequence  A single base substitution is evident in sickle cell anaemia.  Normal Hb has 2 alpha chains and 2 beta chains Normal Hb - Thr-Pro-Glu-Glu-Lys-Ala GAG Sickle cell Hb - Thr-Pro-Val-Glu-Lys-Ala GUG   Red blood cells carrying HbS behave normally when there is an ample supply of oxygen. Reduced oxygen concentration results in the rbc becoming sickle-shaped  Sickled cells may clog capillaries  The body’s defenses destroy the clogging cells and the loss of blood cells causes anaemia  https://youtu.be/yobHMYhHAXE Frameshift Mutation Genetic Flow
… is the flow of genetic material form DNA to proteins
It is described as the central dogma DNA Replication
This is a duplication of genetic material
During replication the hydrogen bonds holding the nitrogenous bases together are broken by helicase causing separation of the strands
Each separated strand serves as a template for the synthesis of a new strand that is complementary to the parent strand
The enzyme DNA polymerase moves along each template of the open helix reading the nucleotide in the template. One strand acts as the leading strand and the other the lagging strand
The enzyme then joins the complementary nucleotide to the new strand DNA Replication
DNA polymerase is only able to move in the 3' → 5' direction, therefore the enzyme moves in the opposite direction along the two strands
(Remember the strands run opposite to each other, i.e. one strand runs in the 3' → 5' direction and the other in the 5' → 3' direction
Other proteins are needed in the process -: (a) to unwind the helix (b) to keep the strands separated (c) to join the segments together after into a continuous strand DNA Replication
When the process is completed there would be two identical molecules of double stranded DNA
Each molecule will contain one strand that was obtained from the parent strand
This form of DNA replication is described as semi conservative because half of the DNA molecule comes from the parent http://www.uic.edu/classes/phar/phar331/lecture4/replication2.jpg DNA Replication
The DNA polymerase is a phenomenal enzyme, in that it is able to reduce the number of mistakes made in complementary base pairing
The enzyme contains two active sites i.e. one for polymerization and the other for proof reading
If a strand is being synthesized and a wrong nucleotide is selected by the first active site then the second active site would recognize the error and remove the incorrect nucleotide
If the second active site does not recognize the error then this results in a permanent change or genetic mutation Transcription
This is the process by which RNA is formed from DNA
The information stored in the DNA molecule is carried by the mRNA molecule
During transcription the double helix of the DNA temporarily separates
A complementary strand of mRNA assembles on one of the DNA strand (template/antisense strand)
The process is catalyzed by RNA polymerase Transcription
The RNA strand is synthesized in the 5' → 3' direction
At the end of the process, the mRNA will contain the complementary genetic information of the DNA
The mRNA then leaves the DNA template where it carries the information to the ribosomes so the synthesis of polypeptides can take place
N.B mRNA is the only RNA synthesized by a cell http://fig.cox.miami.edu/~cmallery/150/gene/c7.17.7b.transcription.jpg Translation
Every three consecutive nucleotide on the mRNA is called a codon
Each codon codes for a particular amino acid
The mRNA determines the sequence of amino acids in a protein
This occurs in a process called translation
It is the most complex process of the cell
The process requires numerous enzymes, ribosomes, amino acids, mRNA, tRNA and energy (ATP and GTP) Translation
One of the folds on the tRNA molecule has a specific triplet codon to which an amino acid is attached
The amino acid binds covalently to this region
At least one kind of tRNA is present for each of the 20 amino acid
Some amino acids have more than one tRNA molecules Translation
There is a triplet codon called an anticodon located at another folded end of the tRNA molecule
The anticodon is the complementary code for the amino acid attached
E.g. tRNA with valine (GUA) attached will have an anticodon CAU When the amino acid is linked to the tRNA molecule, base pairing can occur between the anticodon region of the tRNA and the mRNA molecule Functional ribosome http://genomebiology.com/content/figures/gb-2003-4-12-237-1.jpg Translation
Each triplet codon on the mRNA specifies the insertion of a particular amino acid
The tRNA carrying the appropriate amino acid can become attached to the mRNA
Therefore the message on the mRNA is read codon by codon until the synthesis of a polypeptide chain is completed
Translation involves three main steps (a) Initiation (b) Elongation (c) Termination Initiation
During the initiation step, the small subunit of the ribosome binds at the start codon (AUG) near the 5' end of the strand
It is then joined by the large subunit of the ribosome and a special initiator tRNA molecule (i.e. one that codes for met)
If the message is read at the wrong nucleotide in the start sequence, then the remaining triplets would be incorrectly read
The wrong amino acid would be inserted producing a useless polypeptide Initiation e.g. correct a.a. order - met leu mRNA sequence - AUG CUG mRNA sequence - AUG CUG Incorrect a.a. order - ala his pro CAU CCA CAU CCA ala ser Elongation
A tRNA with the amino acid bonded to it then base pairs with the mRNA molecule
This process requires energy
The preceding amino acid (met) is then linked to the incoming amino acid by a peptide bond
The initiator tRNA to which methionine was attached is then released
The ribosome then moves to the next codon where base pairing between tRNA and mRNA molecule occurs Termination
The end of the translation occurs when the ribosome reaches a stop codon (i.e. either UAA, UAG, UGA)
There exists no tRNA molecule with anticodons for stop codons. Hence there exists no amino acids that codes for these codons
Release factors recognize these codons and releases the polypeptide chain from the ribosome…the process requires energy
The ribosome then split into its subunit which can be reassembled later for another round of protein synthesis http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Translation.gif Translation
Protein synthesis is an efficient process
Many mRNA molecules can be translated at the same time as there exist numerous ribosomes