Dna Translation And Transcription Pdf
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Protein biosynthesis or protein synthesis is a core biological process, occurring inside cells , balancing the loss of cellular proteins via degradation or export through the production of new proteins. Proteins perform a number of critical functions as enzymes , structural proteins or hormones. Protein synthesis is a very similar process for both prokaryotes and eukaryotes but there are some distinct differences.
Protein synthesis can be divided broadly into two phases - transcription and translation. This conversion is carried out by enzymes, known as RNA polymerases , in the nucleus of the cell. The mature mRNA is exported from the cell nucleus via nuclear pores to the cytoplasm of the cell for translation to occur. During translation, the mRNA is read by ribosomes which use the nucleotide sequence of the mRNA to determine the sequence of amino acids.
The ribosomes catalyze the formation of covalent peptide bonds between the encoded amino acids to form a polypeptide chain. Following translation the polypeptide chain must fold to form a functional protein; for example, to function as an enzyme the polypeptide chain must fold correctly to produce a functional active site. In order to adopt a functional three-dimensional 3D shape, the polypeptide chain must first form a series of smaller underlying structures called secondary structures.
The polypeptide chain in these secondary structures then folds to produce the overall 3D tertiary structure. Once correctly folded, the protein can undergo further maturation through different post-translational modifications. Post-translational modifications can alter the protein's ability to function, where it is located within the cell e.
Protein biosynthesis has a key role in disease as changes and errors in this process, through underlying DNA mutations or protein misfolding, are often the underlying causes of a disease. Mutations can cause the polypeptide chain to be shorter by generating a stop sequence which causes early termination of translation.
Alternatively, a mutation in the mRNA sequence changes the specific amino acid encoded at that position in the polypeptide chain. This amino acid change can impact the proteins ability to function or to fold correctly. These clumps are linked to a range of diseases, often neurological , including Alzheimer's disease and Parkinson's disease. However, in prokaryotes post-transcriptional modifications are not required so the mature mRNA molecule is immediately produced by transcription.
Initially, an enzyme known as a helicase acts on the molecule of DNA. DNA has an antiparallel , double helix structure composed of two, complementary polynucleotide strands, held together by hydrogen bonds between the base pairs. The helicase disrupts the hydrogen bonds causing a region of DNA - corresponding to a gene - to unwind, separating the two DNA strands and exposing a series of bases. Despite DNA being a double stranded molecule, only one of the strands acts as a template for pre-mRNA synthesis - this strand is known as the template strand.
The other DNA strand which is complementary to the template strand is known as the coding strand. This property of directionality is due to the asymmetrical underlying nucleotide subunits, with a phosphate group on one side of the pentose sugar and a base on the other.
The five carbons in the pentose sugar are numbered from 1' where ' means prime to 5'. Therefore, the phosphodiester bonds connecting the nucleotides are formed by joining the hydroxyl group of on the 3' carbon of one nucleotide to the phosphate group on the 5' carbon of another nucleotide.
Hence, the coding strand of DNA runs in a 5' to 3' direction and the complementary, template DNA strand runs in the opposite direction from 3' to 5'. The enzyme RNA polymerase binds to the exposed template strand and reads from the gene in the 3' to 5' direction. Simultaneously, the RNA polymerase synthesizes a single strand of pre-mRNA in the 5'-to-3' direction by catalysing the formation of phosphodiester bonds between activated nucleotides free in the nucleus that are capable of complementary base pairing with the template strand.
Despite the fast rate of synthesis, the RNA polymerase enzyme contains its own proofreading mechanism. The proofreading mechanisms allows the RNA polymerase to remove incorrect nucleotides which are not complementary to the template strand of DNA from the growing pre-mRNA molecule through an excision reaction. Therefore, in the pre-mRNA molecule, all complementary bases which would be thymine in the coding DNA strand are replaced by uracil.
Once transcription is complete, the pre-mRNA molecule undergoes post-transcriptional modifications to produce a mature mRNA molecule. The 5' cap is added to the 5' end of the pre-mRNA molecule and is composed of a guanine nucleotide modified through methylation. The purpose of the 5' cap is to prevent break down of mature mRNA molecules before translation, the cap also aids binding of the ribosome to the mRNA to start translation  and enables mRNA to be differentiated from other RNAs in the cell.
Genes are composed of a series of introns and exons , introns are nucleotide sequences which do not encode a protein while, exons are nucleotide sequences that directly encode a protein. Introns and exons are present in both the underlying DNA sequence and the pre-mRNA molecule, therefore, in order to produce a mature mRNA molecule encoding a protein, splicing must occur.
During translation, ribosomes synthesize polypeptide chains from mRNA template molecules. In eukaryotes, translation occurs in the cytoplasm of the cell, where the ribosomes are located either free floating or attached to the endoplasmic reticulum. In prokaryotes, which lack a nucleus, the processes of both transcription and translation occur in the cytoplasm. Ribosomes are complex molecular machines , made of a mixture of protein and ribosomal RNA , arranged into two subunits a large and a small subunit , which surround the mRNA molecule.
The ribosome reads the mRNA molecule in a 5'-3' direction and uses it as a template to determine the order of amino acids in the polypeptide chain. Each tRNA is composed of nucleotides and adopts a characteristic cloverleaf structure due to the formation of hydrogen bonds between the nucleotides within the molecule.
There are around 60 different types of tRNAs, each tRNA binds to a specific sequence of three nucleotides known as a codon within the mRNA molecule and delivers a specific amino acid. The mRNA nucleotide sequence is read in triplets - three adjacent nucleotides in the mRNA molecule correspond to a single codon.
Each tRNA has an exposed sequence of three nucleotides, known as the anticodon, which are complementary in sequence to a specific codon that may be present in mRNA. For example, the first codon encountered is the start codon composed of the nucleotides AUG. This tRNA delivers the correct amino acid corresponding to the mRNA codon, in the case of the start codon, this is the amino acid methionine. The next codon adjacent to the start codon is then bound by the correct tRNA with complementary anticodon, delivering the next amino acid to ribosome.
The ribosome then uses its peptidyl transferase enzymatic activity to catalyze the formation of the covalent peptide bond between the two adjacent amino acids. The ribosome then moves along the mRNA molecule to the third codon. The ribosome then releases the first tRNA molecule, as only two tRNA molecules can be brought together by a single ribosome at one time.
The next complementary tRNA with the correct anticodon complementary to the third codon is selected, delivering the next amino acid to the ribosome which is covalently joined to the growing polypeptide chain. This process continues with the ribosome moving along the mRNA molecule adding up to 15 amino acids per second to the polypeptide chain.
Behind the first ribosome, up to 50 additional ribosomes can bind to the mRNA molecule forming a polysome , this enables simultaneous synthesis of multiple identical polypeptide chains. When this occurs, no tRNA can recognise it and a release factor induces the release of the complete polypeptide chain from the ribosome. Once synthesis of the polypeptide chain is complete, the polypeptide chain folds to adopt a specific structure which enables the protein to carry out its functions.
The basic form of protein structure is known as the primary structure , which is simply the polypeptide chain i. The primary structure of a protein is encoded by a gene. Therefore, any changes to the sequence of the gene can alter the primary structure of the protein and all subsequent levels of protein structure, ultimately changing the overall structure and function.
The primary structure of a protein the polypeptide chain can then fold or coil to form the secondary structure of the protein. The most common types of secondary structure are known as an alpha helix or beta sheet , these are small structures produced by hydrogen bonds forming within the polypeptide chain.
This secondary structure then folds to produce the tertiary structure of the protein. The tertiary structure is the proteins overall 3D structure which is made of different secondary structures folding together. In the tertiary structure, key protein features e. Finally, some proteins may adopt a complex quaternary structure. Most proteins are made of a single polypeptide chain, however, some proteins are composed of multiple polypeptide chains known as subunits which fold and interact to form the quaternary structure.
Hence, the overall protein is a multi-subunit complex composed of multiple folded, polypeptide chain subunits e. When protein folding into the mature, functional 3D state is complete, it is not necessarily the end of the protein maturation pathway. A folded protein can still undergo further processing through post-translational modifications. There are over known types of post-translational modification, these modifications can alter protein activity, the ability of the protein to interact with other proteins and where the protein is found within the cell e.
There are four key classes of post-translational modification: . Cleavage of proteins is an irreversible post-translational modification carried out by enzymes known as proteases.
These proteases are often highly specific and cause hydrolysis of a limited number of peptide bonds within the target protein. The resulting shortened protein has an altered polypeptide chain with different amino acids at the start and end of the chain. This post-translational modification often alters the proteins function, the protein can be inactivated or activated by the cleavage and can display new biological activities.
Following translation, small chemical groups can be added onto amino acids within the mature protein structure. Methylation is the reversible addition of a methyl group onto an amino acid catalyzed by methyltransferase enzymes. Methylation occurs on at least 9 of the 20 common amino acids, however, it mainly occurs on the amino acids lysine and arginine. One example of a protein which is commonly methylated is a histone. Histones are proteins found in the nucleus of the cell.
DNA is tightly wrapped round histones and held in place by other proteins and interactions between negative charges in the DNA and positive charges on the histone.
A highly specific pattern of amino acid methylation on the histone proteins is used to determine which regions of DNA are tightly wound and unable to be transcribed and which regions are loosely wound and able to be transcribed. Histone-based regulation of DNA transcription is also modified by acetylation.
Acetylation is the reversible covalent addition of an acetyl group onto a lysine amino acid by the enzyme acetyltransferase. The acetyl group is removed from a donor molecule known as acetyl coenzyme A and transferred onto the target protein. The effect of acetylation is to weaken the charge interactions between the histone and DNA, thereby making more genes in the DNA accessible for transcription. The final, prevalent post-translational chemical group modification is phosphorylation.
Phosphorylation is the reversible, covalent addition of a phosphate group to specific amino acids serine , threonine and tyrosine within the protein. The phosphate group is removed from the donor molecule ATP by a protein kinase and transferred onto the hydroxyl group of the target amino acid, this produces adenosine diphosphate as a biproduct.
This process can be reversed and the phosphate group removed by the enzyme protein phosphatase. Phosphorylation can create a binding site on the phosphorylated protein which enables it to interact with other proteins and generate large, multi-protein complexes.
Alternatively, phosphorylation can change the level of protein activity by altering the ability of the protein to bind its substrate. Post-translational modifications can incorporate more complex, large molecules into the folded protein structure. One common example of this is glycosylation , the addition of a polysaccharide molecule, which is widely considered to be most common post-translational modification. In glycosylation, a polysaccharide molecule known as a glycan is covalently added to the target protein by glycosyltransferases enzymes and modified by glycosidases in the endoplasmic reticulum and Golgi apparatus.
Glycosylation can have a critical role in determining the final, folded 3D structure of the target protein. In some cases glycosylation is necessary for correct folding.
DNA deoxyribonucleic acid is one of the most important molecules in your body, and though around Did you know that in the average human cell, there is about 2m 6ft of DNA? How is all that genetic material packed into a space way smaller than the head of a pin? The short answer is a whole lot of twisting and winding. DNA wraps around protein clusters called histones to form units called nucleosomes. These nucleosomes fold into a zig-zag patterned fiber, which then forms loops.
1: DNA Replication, Transcription and Translation
Transcription begins with a bundle of factors assembling at the promoter sequence on the DNA in red. Here, two transcription factors are already bound to the promoter. Other proteins arrive, carrying the enzyme RNA polymerase in blue.
Function: DNA base sequence encodes information for amino acid sequence of proteins. Genetic code: 1 to 1 relationship between a codon specific sequence of 3 bases and 1 amino acid. DNA Structure: figure 8.
Pathway for biological sciences education. For more teaching resources, please visit BEN to use their searchable database. BEN is free to use, but requires registration. Your Account. Author Profile.
Stay informed! Sign up for our newsletter. We will never send you spam or sell your information. Creating an account will give you access to additional content and tools. This hands-on activity reinforces the processes of transcription and translation. Usingpaper cut-outs, students follow the rules of complementary base pairing to build an mRNAmolecule, then translate the mRNA codons to assemble amino acids, building a protein.
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