5.1: The Genetic Code and Cell Function Flashcards
Where does the process of D.N.A replication occur?
D.N.A replication takes place in the nucleus of a cell just before it divides.
What is D.N.A replication?
This is when D.N.A makes copies of itself.
For D.N.A replication to occur, what two things must be present in the nucleus?
Free (non-bonded) D.N.A. Nucleotides of the four types (A, T, C and G) An enzyme called D.N.A polymerase.
What are the three functions of D.N.A polymerase?
It progressively breaks down Hydrogen bonds between the base pairs, exposing them to the nucleus - D.N.A is unzipped. Following the base pairing rule it bonds free nucleotide bases with exposed nucleotide bases. -Ensuring that the correct base pairing takes place. It bonds pentose sugars to phosphates to form the sugar/phosphate backbone.
What is the end result of D.N.A replication?
The creation of two genetically identical D.N.A molecules. (I.e the base sequence is identical)
Outline the Semi-conservative D.N.A replication theory.
This hypothesises that D.N.A replicates in the following manner: 1.) D.N.A polymerase moves along the D.N.A molecule, unzipping it. 2.) The two unzipped strands now act as a template to which D.N.A polymerase bonds together free nucleotide phosphates according to the base pairing rule. This results in the creation of two identical D.N.A molecules, each containing a single strand of the original molecule.
Who created the theory of Semi-conservative D.N.A replication?
Meselson and Stahl.
His did Meselson and Stahl come up with their theory?
By looking at E.coli which has a generation time of 50 minutes at 36°C (I.e D.N.A replicates every 50 minutes) . They allowed E.coli to grow in a solution containing only the heavy Nitrogen isotope - ¹⁵N. Because of this the bacteria only incorporated ¹⁵N in their D.N.A and the bacteria had become “radioactively labeled”. The radioactive bacteria were then transferred into in environment only containing ¹⁴N. From this point on D.N.A samples were taken every 50 minutes and spun in a centrifuge containing CsCl for 20 hours. Under these conditions D.N.A sediments out depending on its density (I.e ¹⁵N is heavier than ¹⁴N D.N.A)
What were the results of Meselson’s and Stahl’s experiment?
As generations progressed a smaller and smaller proportion of D.N.A consisted of ¹⁴N/¹⁵N D.N.A and a greater proportion consisted of ¹⁴N D.N.A
What is the purpose of the Genetic code?
The purpose of the D.N.A base code is to represent the sequence of amino acids in a polypeptide chain.
What is the triplet code?
This means that each amino acid is coded for by three Bases.
What evidence is there for the triplet code?
A three base code is used is because a two base code I.e 4² can code for only 16 amino acids. While a one base code can code for only four amino acids.
What is a codon?
A base triplet which codes for a single amino acid.
In what respect is the genetic code universal?
In the respect that each amino acid is coded for by the same codon(s) in every living thing on earth.
Define a gene.
A sequence of D.N.A bases beginning at a start codon and ending at a stop codon and specifying the position of amino acids in a polypeptide chain.
What is transcription?
The process of copying a gene to produce m.R.N.A
What does transcription require?
R.N.A nucleotides (ribonucleoside triphosphates) and R.N.A polymerase. (D.N.A dependant R.N.A-polymerase)
What is the function of D.N.A dependant R.N.A-polymerase in transcription?
This enzyme operates as a code reader allowing the coding strand for one gene to be copied into R.N.A language.
What is the first stage in Transcription?
R.N.A polymerase locks on to the start codon of a gene.
What is the second stage in transcription?
there is space inside the enzyme for two codons and the enzyme breaks the Hydrogen bonds between the coding and non-coding bases in these two codons.
What is the third stage in transcription?
Breaking the Hydrogen bonds between the coding and non-coding strands exposes two D.N.A codons within the enzyme which are now bonded to R.N.A nucleotides using the base pairing rule.
What is the fourth stage in transcription?
These six R.N.A nucleotides are now sugar/phosphate bonded (phospho-ester link) before the hydrogen bonds securing them in place are broken.
What is the fifth stage in transcription?
The formation of the phospho-ester link allows the enzyme to move forward by one codon, exposing another three D.N.A bases and allowing the next R.N.A codon to be positioned.
What is the sixth stage in transcription?
the process continues along the entire length of the gene until the enzyme reaches the stop codon. At which point the enzyme detaches from the coding strand and the single stranded mR.N.A copy of the gene is detached.
What are introns?
Over time genes accumulate junk coding I.e D.N.A sequences which have no function. These are called introns.
Why must introns be removed from m.R.N.A before translation?
Unless these are removed the m.R.N.A molecule created will code for the wrong protein or a functionless protein.
What is the structure of t.R.N.A?
Each t.R.N.A has a different anti-codon depending on the amino acid it carries. The bonding of amino acids with t.R.N.A is an enzyme-controlled system which takes place in the cytoplasm.
