KEGS Biology Blog

YEAR 13 Tuesday 20th November F215
November 20, 2012, 9:11 am
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Explaining the term ‘Genetic control’ and DNA conference


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Notes on the following specification points:

(a) State that genes codes for polypeptides, including enzymes:

The following are definitions:

Gene – is a functional unit of a length of DNA that codes for one (or more) polypeptide chains. This means that the sequence of nucleotide bases it represents will form a particular polypeptide during protein synthesis.
Polypeptide – is a polymer consisting of a chain of amino acid residues joined by peptide bonds.
Protein – is a long chain polypeptide – usually around 100 amino acids. Proteins consist of one or more polypeptide chains.
Enzymes – are biological molecules that act as metabolic catalysts. Most (if not all) enzymes are proteins.

From these definitions it is clear that if an enzyme is a protein, a protein is a long polypeptide and a gene codes for a polypeptide then the specification point has been proven.

(b) Explain the meaning of the term genetic code:

The genetic code of an organism is the sequence in which the nucleotide bases appear in (to form genes). A good website that can explain this in more detail is:

The Genome is the entire DNA sequence of a particular organism – a human genome can consist of around 3 billion base pairs. The genome is used to identify genetic diseases and also to see how closely we are evolved from different organisms. (Though most of the genome is bases that do not code for genes – effectively empty space)

– Triplet code. So a sequence of 3 nucleotide bases codes for an amino acid. As there are 4 bases arranged into groups of 3, the number of possible sequences available is 43 = 64. But as there are only 20 amino acids used for protein synthesis this is more than enough/
– Degenerate code. ALL amino acids but methionine have more than one code.
– Some codes indicate a “stop” code to stop the polypeptide chain.
– It is not universal – variations of amino acids within living organisms means that there may be more than one code for a particular amino acid or more than one code for a stop code.

Comment by erinmeredithh

-State that genes code for polypeptides, including enzymes;
A gene is a length of DNA. This means it is a sequence of nucleotide bases that codes for one or more polypeptides, including enzymes.

-Explain the meaning of the term ‘genetic code’
The role of DNA is to instruct the cell to make specific proteins. Information in DNA determines the sequence of amino acids of the proteins which the cell manufactures ceaselessly. This information exists in the sequence of bases in DNA and is known as the genetic code.

Proteins are made from some twenty different types of amino acid. An average protein contains several hundred amino acids, condensed together in a specific sequence. DNA consists of only four different bases. Thus there are only four ‘letters’ to the base code ‘alphabet’ with which to code for all twenty amino acids. Combinations of three bases code for the amino acids and this is known as the ‘triplet code’.

The code was eventually broken by the preparation of lengths of nucleic acid in which one triplet code was repeated many times. Subsequently, synthetic messenger RNAs with various known sequences of nucleotides were produced, enabling the complete code to be deciphered. These synthetic nucleic acids (messenger RNA) were added to cell-free protein-synthesising systems containing all twenty amino acids and other metabolites and the polypeptides formed were analysed.

The genetic code is a listing of all the triplets of mRNA. In the code there are more triplets than there are amino acids so the code is described as degenerate. All amino acids except methionine have more than one code. In effect, the code contains more ‘words’ than are required, and the spare capacity in the language is used rather than left unquoted. Some of the words code for a ‘full stop’, rather than for another amino acid. These are the protein chain-terminating codes.

Another important discovery is that the code is widespread. This has proved useful for genetic engineering as we can transfer genes from one organism into another to produce the same protein. However there are some variations. In mammalian mitochondria there are two codes for methionine, and one of the standard stop codes for tryptophan. In ciliated protoctists, two of the standard stop codes code for glutamic acid.

Comment by ebelen

a) State that genes code for polypeptides including enzymes;

A gene is a length of DNA which codes for a specific polypeptide, or in some cases, multiple polypeptides.

b) Explain the meaning of the term genetic code;

The genetic code is the sequence of nucleotide bases on a gene that codes for the construction of a certain polypeptide. The genetic code has a few characteristics.

-Firstly, it’s a triplet code. This means that three nucleotide bases will code for one amino acid.

-Secondly, it’s a degenerate code which means that all amino acids (other than methionine in humans) have more than one code.

-Thirdly, not all codes will correspond to an amino acid. Some correspond to STOP codes, which indicate the end of the polypeptide chain

-Finally, the genetic code is widespread but not universal. By this we mean that it is used nearly everywhere in the animal kingdom but it is not identical across all species.

Notes from today
– DNA bases are read in groups of 3, called a triplet code. Each triplet sequence codes for a different amino acid.
– Some amino acids are coded for by many different codes e.g. CGA, CGC, CGT all code for Valine.
– Some triplet codes are for STOP codons- to stop the translation process.

Comment by hamzamahmud

Hey team, thought it would be nice to briefly summarise one of the talks some of us received at the Institute of Education last Tuesday, as it loosely concerns what we are studying currently (and will be studying in the future). The talk was on Epigenetics, which is a big thing at the moment in the world of biology. Essentially, Epigenetics is the study of (inherited) changes in phenotype or gene expression caused by things other than changes in the DNA sequence of an organism. The simplified definition, which the speaker gave, is genetic identity ≠ phenotypic identity. The speaker then went on to give an example of this: crocodiles; male and female crocodiles have the same DNA sequence, but the eggs are fertilised differently (can’t exactly remember how, believe it was something to do with the eggs being fertilised at different temperatures), leading to different genders.

Epigenetics also concerns the idea of genetic switches, which are molecules that chemically modify histones or DNA, and therefore increase/decrease the liklihood of regions of DNA being expressed (without changing the genetic code). This, according to a BBC article I found on the interweb, will change the way the causes of disease are viewed. I imagine that now, scientists and clinicians alike may believe that with some (maybe all) human diseases, there is nothing wrong with the genetic information in the cells of the patient, but instead there is an epigenetic modification, which has changed the expression of a normal pattern, leading to said disease. Applying this to an example, cancer, a disease of failure of regulation of tissue growth. Potentially, the genes which regulate cell growth and differentiation (e.g., the tumour suppressor gene?) may have nothing (in terms of genetic sequence) wrong with them, it’s just that certain regions of the gene are being expressed more than others, leading to the malignant neoplasm.

Hope you found this interesting and/or helpful. I will attach below a link for a Guardian article, talking about a story concerning switches, from September.

Comment by benjamincharlespatterson

(a) State that genes code for polypeptides, including enzymes.

Genes can be defined as segments of DNA containing a sequence of nucleotides which express an inherited trait.

Polypeptides are the product of numerous amino acids joined together via peptide bonds in a large polymer.

(b) explain the meaning of the term genetic code

At its simplest, the genetic code is the information encoded within genes. This information then translated into polypeptides or proteins. The genetic code comprises of so called ‘codons’ which are triplets of nucleotides organised in a particular unique fashion, thus generating a section of the genetic code. There are four organic nitrogenous bases found in DNA and consequently there will be a total of 64 unique permutations. To further this, theoretical physicist and cosmologist George Gamow proposed the theory that three of these are taken in combination to encode an amino acid, relating directly to the triplets or codons mentioned. However, he postulated an incorrect theory of overlapping triplets when in fact the genetic code is degenerate.

One result of the degeneracy of the genetic code is that of synonymous substitution, which is defined as a change in the base pair of a gene alone, not affecting the overall specific sequence of amino acids in the protein. It is a common misconception that synonymous substitution and silent mutation are interchangeable but this is fundamentally incorrect. Silent mutations are characterised by their base change both within exons and non-coding regions. However synonymous substitution is only within exons despite being a mutation in its own right.

Tryptophan (UGG) and the initiation amino acid methionine (AUG) are the only amino acids which have only one defined code. Methionine is the so called initiator of the protein translation due to the fact that it is the codon which sends such a signal to the ribosome and mRNA to stimulate translation. However conversely, there are three termination codons which stop the translation process, facilitated by release factors which are recognised, causing cleavage of the peptidyl-tRNA bond via hydrolysis causing the protein to be released.

The genetic code is also relatively universal; many hypotheses trace this fact back to evolutionary roots, namely the biosynthetic expansion theory (the endosymbiont theory partakes a similar route, you may be more familiar with that one) which argues that amino acids were found as possible by-products but were proved to be useful in the base genetic code and were henceforth encorporated into the early organisms.

Comment by hasssankhan

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