GENETIC CODE
dr. I Gde Haryo Ganesha, S.Ked Dept. of Medical Education Maria Septiana Parmonang Aroean
Angkatan 2016 (1602511167)
FACULTY OF MEDICINE UDAYANA UNIVERSITY
2016
ii TABLE OF CONTENTS
TABLE OF CONTENTS ... ii
CHAPTER I INTRODUCTION ...1
1.1 Background ...2
1.2 Problem Statement ...2
1.3 Purpose ...2
1.4 Benefit ...3
CHAPTER II CONTENT ...4
2.1 Definition ...4
2.2 Characteristic of Genetic Code ...4
2.3 Gene Expresion ...6
2.4 Function of Genetic Code ...9
2.5 Mutation Gen and Disease ...9
CHAPTER III SUMMARY ...12 REFERENCES
1 CHAPTER I
INTRODUCTION
1.1 BACKGROUND
The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells. Translation is accomplished by the ribosome, which links amino acids in an order specified by mRNA, using transfer RNA (tRNA) molecules to carry amino acids and to read the mRNA three nucleotides at a time. The genetic code is highly similar among all organisms and can be expressed in a simple table with 64 entries. The code defines how sequences of nucleotide triplets, called codons, specify which amino acid will be added next during protein synthesis.A three-nucleotide codon in a nucleic acid sequence specifies a single amino acid. Because the vast majority of genes are encoded with exactly the same code (see the RNA codon table), this particular code is often referred to as the canonical or standard genetic code, or simply the genetic code, though in fact some variant codes have evolved. For example, protein synthesis in human mitochondria relies on a genetic code that differs from the standard genetic code.While the "genetic code"
determines a protein's amino acid sequence, other genomic regions determine when and where these proteins are produced according to a multitude of more complex "gene regulatory codes".
Codon (genetic code) is a series of nucleotides in the mRNA which consists of a combination of three-nucleotide sequence that encodes a particular amino acid, so often referred to as a triplet codon. The amino acids encoded by the example of methionine by the nucleotide sequence ATG (AUG in RNA). Many amino acids are encoded by more than one codon. Codons are in an mRNA molecule. Translation of mRNA into protein-coding is done on roads flanked by the start codon (AUG) and the final codon (UAA, UAG, or UGA), these segments called genes.
Codon in an mRNA molecule can encode amino acids with the help of interpretation codon by tRNA. Each tRNA carries the amino acid corresponding to three or triplet nucleotide sequence called anticodon that are in the tRNA anticodon node. Anticodon binds complementarily at codon in mRNA, so that amino acids brought by the tRNA in accordance with existing codon in mRNA. The genetic message translation codon by codon in a way tRNA carries amino acids
2 corresponding anticodon complementary to a codon and ribosome connect amino acids into a polypeptide chain. Ribosome adds each amino acid carried by tRNA to the end of the growing polipeptide chain.
The genetic code codes for proteins. The information of the DNA is ‘translated’ into a chain of amino acids that forms a protein. These proteins form the building blocks for structures within the cells and ultimately the whole body. Proteins also form enzymes and other chemicals that perform various functions in the body. Each gene can code for different proteins and thus the number of proteins known to exist in the cells is more than the number of genes. All genes are not expressed or do not code for any protein. This could be organ specific for example a liver cell expresses different genes than kidney cells. The environment also plays a role in determining the ultimate trait. The phenotype of an organism thus depends on the interaction of genetics with the environment. The environment for example, has a role in effects of the human genetic disease phenylketonuria. The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine. This leads to toxic build-up of an intermediate molecule leading to mental retardation and seizures. Persons with phenylketonuria mutation on a strict diet that avoids this amino acid may remain normal and healthy.
1.2 PROBLEM
1. What is the genetic code?
2. What are properties of the genetic code?
3. What is meant by the start codon and stop codon?
4. How the process of protein synthesis?
5. What are the functions of the genetic code?
6. What is relation between mutation and genetic code?
1.3 PURPOSE
1. To know the genetic code
2. To know properties of the genetic code
3. To know meant by the start codon and final codon 4. To know the process of protein synthesis
3 5. To know the functions of the genetic code in the body
6. To know the relation between mutation and genetic code
1.4 BENEFIT
1. Giving additional information to public about genetic code
4 CHAPTER II
CONTENT
2.1 DEFINITION OF GENETIC CODE
Genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into protein by living cells. This help in determining the amino acid sequence used in the synthesis of an organism proteins and it is universal in all organism. Genetic code composed by codon, codon is a sequence of three DNA or RNA nucleotides that corresponds with specific amino acid or stop signal during protein synthesis.
2.2 CHARACTERISTIC OF GENETIC CODES Characteristic of genetic codes are:
- Triplet code
The group bases specifying one amino acid is called a codon or core word. The codons are formed using the bases available in mRNA. There are strong evidences to prove that a sequence of three nucleotides code for one amino and protein, the code is triplet.
The four nucleotide bases (A, G, C, and U) in mRNA are used to produce the three base codons. The 64 codons include the sense codons (codons that specify amino acid). There are therefore, 64 codon code for the 30 amino acids, and since each codon code for only one amino acids this means that, there are more than one code for the same amino acid.
- Commaless code
There is no punctuation between; that is, each codon is immediately adjacent to the next, without any spacer nucleotides in between.
5 - Nonoverlapping code
The code is sequentially read in group of three. A nucleotide that forms part of a triplet never forms part of the next triplet. Each triplet is read from 5’ -> 3’ direction so the first base is 5’ base, followed by the middle base then the last base which is 3’ base.
Examples:
5’-AUG-3’ codes for methionine 5’-UCU- 3’ codes for serine 5’-CCA-3’ codes for proline
- The coding dictionary
Picture 1. The Codon Table - Degenerate code
All amino acids except methionine (AUG) and tryptophan (UGG) are coded by several codons: that is; some codons are synonyms. For example, theonine is coded by four codons ACU, ACC, ACA and ACG.
6 - Universality of code
Genetic code is largely universal for all living organisms and viruses. However a few exceptions are found in mitochondria. For example, UGA, one of the termination codons in human, is the code for tryptophan yeast in mitochondria. Few other examples are stated below:
- Non ambiguous code
The code is unambiguous under normal conditions, that means that each codon specifies the same amino acid all the time.
- Chain initiation codons
The initiation signal for the synthesis polypeptide chain is AUG - Chain termination codons
The termination signal is provided by three codons UAG(Amber), UAA (ochre) and UGA (opal). These chain termination codons do not code for any amino acids and hence termed as non sense codons.
- Polarity
The genetic code has polarity, the code is always read in fixed direction, in the 5’ to 3’ direction. It is apparent that if the code is read in opposite direction, it would specify 2 different protein.
2.3 GENE EXPRESSION
The process of gene expression simply refers to the events that transfer the information content of the gene into the production of a functional product, usually a protein. Although there are genes whose functional product is an RNA, including the genes encoding the ribosomal RNAs as well as the transfer RNAs and certain other small RNAs, the vast majority of genes within the cell are protein-encoding genes. Mechanism of gene expression are as follows:
7 - Transcription
The initial step in gene expression is the transcription of the DNA molecule into an exact RNA copy. The transfer of information, to the ultimate synthesis of a protein, is accomplished via an RNA intermediate, the so-called messenger RNA (mRNA). The mRNA molecule contains the exact same sequence of nucleotides as found in the DNA molecule (with U substituted for T).
This occurs through the process known as transcription and is carried out by an enzyme termed DNA-dependent RNA polymerase. Transcription always proceeds in a 5' to 3' direction with respect to polarity of the nucleotides in the RNA. Thus, an unmodified primary transcription product would contain a 5' end with a triphosphate and a 3' end with a OH.
The polymerase must initiate transcription. This does not involve a primer molecule as is the case for DNA synthesis but rather starts at a specific site in the gene. This site is dictated as a result of the interaction of the RNA polymerase at a specific site, guided by the transcription factors that have bound to the promoter and enhancer sequences. The polymerase must complete the transcription of the gene and then terminate transcription. In some cases, the termination of transcription is precise whereas in other cases it can occur heterogeneously over a broad region of DNA.
- Post-Transcriptional Event
Whereas the initial transcript of a bacterial gene is the actual messenger RNA, the initial transcript of a eukaryotic gene must be altered in a variety of ways before it can function. Thus, post-transcriptional processing and modification events are critical to the formation of a eukaryotic mRNA.
- RNA Modifications
In addition to the various steps that process the initial primary transcript, the mRNA is also modified in several ways. The 5' terminus of the transcript is capped by the addition of a modified GTP residue that forms a 5'-5' linkage. Internal adenosine residues in the RNA are modified by methylation but the function of these modifications is not known.
8 - Nucleus-Cytoplasmic RNA Transport
Unlike bacteria, the eukaryotic cell is compartmentalized. Therefore, the final processed product (mRNA) must be transported through the nuclear envelope to reach the cytoplasm and be engaged with the ribosomes for translation.
- Translation
Translation is essentially the reading of the sequence in the mRNA to direct the synthesis of a unique protein. There are two components of translation: Ribosome, a multicomponent RNA-protein structure that serves as the framework upon which protein synthesis takes place and which also provides the enzymatic activity for formation of the peptide bonds; and tRNA, a set of small RNAs, each specific for a given amino acid. The tRNA carries the amino acid to the ribosome for insertion into the growing polypeptide chain. The anticodon in the tRNA is complementary to the codon in the mRNA. The proteins are synthesized from N terminus to C terminus in 3 steps: initiation, elongation, and termination.
a. Initiation
Initiation of protein synthesis involves recognition of a methionine codon (AUG) in the mRNA by a special methionyl tRNA, different from the methionyl tRNA that is used for elongation. The initiation event is accomplished through the action of several initiation factors that allow interaction of the mRNA with the small ribosomal subunit, GTP, and the initiator met-tRNA. The recognition of the initiating AUG codon in the mRNA is facilitated by the RNA sequences that surround the codon. Once the interaction takes place, the large subunit of the ribosome interacts and protein synthesis begins.
b. Elongation
Codons in the mRNA are recognized by tRNAs which carry the appropriate amino acid to the translation machinery. Codon recognition involves base pairing between the codon in the mRNA and the anticodon in the tRNA. There are two functional sites on the ribosome that are occupied by tRNA and that facilitate peptide bond formation. The P site (peptidyl) and the A site (aminoacyl). Following formation of the peptide bond, the tRNA remaining in the P site leaves and the tRNA-peptidyl complex moves to the P site. A new aminoacyl-tRNA, specified
9 by the mRNA codon, then moves into the A site and peptide chain elongation continues. The catalysis of peptide bond formation is a function of the large ribosomal subunit.Very recent evidence indicates that it is the ribosomal RNA component of the large subunit that carries the enzymatic activity of peptidyl transferase.
c. Termination
Three codons (UAG, UAA, and UGA) do not specify an amino acid-tRNA and thus cause termination of translation. These codons signal the release of the peptidyl-tRNA complex when recognized by termination factors. This results in the release of an uncharged tRNA lacking an attached amino acid residue as well as the completed polypeptide chain. The ribosome then disengages from the mRNA and the subunits dissociate, ready to start the cycle over again.
2.4 FUNCTION OF GENETIC CODES
Genetic codes play a role in synthesize of protein. Genetic code becomes a basic to explain how abnormality of proteins can gave some effect such as genetic disorder and we can uphold diagnosis and treatment that should given for genetics disorder. Genetic code that formed by 64 codons, 61 codons bring information of amino acid and three codons induced termination of protein synthesize. We all know that each codon specifies one single acid(Picture1).
Previously we have discuss about characteristic of genetic code, especially every characteristic of genetic code have their function, example to helps prevent effects of DNA mutations, most amino acids have more than one codon, and it is unambiguous that means each codon has only one meaning for one acid.
2.5 GENETIC CODES AND MUTATION
As we know genetic code are composed by codon. Codon is a sequence of three base DNA or RNA and we can also called it as triplet base.
10 A mutation is a change in genetic codes which we can find it in DNA, the hereditary material of life, such that the genetic code differs from what is found in most people, so the wrong genetic codes will occur the mutation. Mutations may or may not cause health problem because it range in size; they can affect anywhere from a base pair to a large segment of a chromosome that includes multiple genes. DNA affects how it looks, how it behaves, and its physiology. So a change in DNA can cause changes in all aspects of its life.
Mutations can be classified in two major ways:
- Hereditary mutations are inherited from a parent and are present throughout a persons life in virtually every cell in the body. They are presenting the parents egg or sperm cells, which are also called germ cells.
- Acquired/somatic mutations occur at some time during a person life and are present only in certain cells. These changes caused by environmental factors or occur if a mistake is made as DNA copies itself during cell division. Acquired mutations in somatic cells cannot be passed on to the next generation.
Mosaicsm is somatic mutation which the genetic changes are not presenting a parent’s egg or sperm cells, or in the fertilized egg, but happen when the embryo growth several cells. As all the cells divide during growth and development, cells that occur from the cell with the altered gene will have the mutation, while other cells will not.Some genetic changes are very rare; others are common in the population. Genetic changes that occur in more than 1% of the population are called polymorphisms. They are common enough to be considered a normal variation in the DNA. Polymorphisms are responsible for many of the normal differences between people such as eye color, hair color, and blood type. Although many polymorphisms have no negative effects on a persons health, some of these variations may influence the risk of developing certain disorder
So as we describe above, the mutation of genetic codes may or may not effect person`s health, it will not damage for examples if the codon UUU becomes UUC, because both of them will produce the same amino acid phenylalanine. But when it comes to a nonsense mutation it may affected, nonsense mutation occur when a codon that supposed to produce amino acid change into stop codon, it will cause some premature protein, which mean the protein is shorter
11 than normal because it can't finish adding all the necessary amino acid and these protein usually are not functional then it may caused many genetic disorder. Then what will happen if the stop codon doesn’t occur? These mRNA that lack of stop codon will cause the translation to continue into the poly-A (lysine). Since there is no stop codon is present, the ribosome attached to the mRNA, it will cause the activation of a pathways known as non-stop decay.
The other changes of genetic code is present in the codon that produce amino acid, when the genetic code is changed then it will produce the different amino acid which mean that the protein are differs from the normal one and it may caused the health problem. There are 4 mutation that will changes the genetic code. Transvertion is the changes pirimidin becomes purin for example if we had the DNA with base TAC it becomes GAC the problem is mRNA will transcript different code and it will leads to different protein. Transition is the change in genetic code where the purin becomes purin and pirimidin becomes pirimidin. Deletion is a change of genetic code caused by deleted some base. And insertion is the change of genetic code because of additional base.
12 CHAPTER III
SUMMARY
Genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into protein by living cells. Genetic code composed by codon, codon is a sequence of three DNA or RNA nucleotides that corresponds with specific amino acid or stop signal during protein synthesis.
There are specific characteristics of genetic codes, such as that a codon that are composed of three nucleotides bases, have a total of 64 for 30 amino acids, therefore there are more than one code for the same amino acid with exception of methionine(AUG) and tryptophan (UGG) that only have one code for each. Another example is that genetic code has polarity and have a main function in synthesizing of protein.
The process of gene expression simply refers to the events that transfer the information content of the gene into the production of a functional product, usually a protein. The mechanism of gene expression are transcription, post-transcriptional event, RNA modification, nucleus- cytoplasmic RNA transport, translation. Meanwhile the process of protein synthesizing follow the order of initiation, elongation, and termination.
A mutation is a change in genetic codes which we can find it in DNA, the hereditary material of life, such that the genetic code differs from what is found in most people. Mutation is classified in two ways, heredity mutation and acquired/somatic mutation. In addition, there are several types of mutations that change genetic codes. There are transversion, transition, deletion and insertion.
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