Genetics- Transcription and Translation

by zeynep_merve10hotmailcouk (student)

Transcription

The transcription phase of the polypeptide synthesis occurs in the nucleus, where the DNA is located. Transcription is constructed of three steps which are Initiation, elongation and termination. Both the nucleic acids (RNA and DNA) are involved in this stage. The first step of transcription is Initiation:

The enzyme RNA polymerase enters the nucleus alongside the messenger RNA (mRNA). RNA polymerase binds to the promoter then begins to unravel the template and coding strand of the DNA from each other by breaking the hydrogen bonds. Only a small section of DNA is unravelled at a time because only one particular gene at a time is to be transcribed onto the specific mRNA strand. The beginning of the sequence for the gene is determined by the promoter (for example, a sequence of bases which are called ‘TATA’ box), which is a region of DNA.

The elongation step involves the reading of the DNA template strand by mRNA. As the RNA polymerase moves down the specific sequence of bases, starting from where it hooked on the DNA via the promoter. It helps the single RNA nucleotides to match up with the nucleotides on the template DNA strand. This process occurs through complementary base pairing, when Adenine matches Uracil, Thymine with Adenine, Guanine with Cytosine and Cytosine with Guanine. The template strand is complementary to the coding strand and due to this reason the RNA strand is built by using the template strand. This way the exact copy of the coding strand is formed on the mRNA strand, the only difference is Uracil takes the place of Thymine.

Once the RNA polymerase reaches the terminator, which is a region of the DNA that signals the end of the elongation step. The enzyme (RNA polymerase) unhooks itself from the template DNA strand. This step is the termination step and is the final one for transcription. The DNA is zipped back into its normal shape with the help of RNA polymerase, and the hydrogen bonds are re-formed between the template and coding strands from the point where it had been unzipped. After this, the formed mRNA strand leaves the nucleus through the nuclear pore.

Translation

The translation ...

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Translation

The translation phase of polypeptide synthesis occurs in the Ribosome and Rough Endoplasmic Reticulum (when ribosomes are found in the structure of Endoplasmic Reticulum), which are located in the cytoplasm. Ribosomes are constructed of a heavy sub-unit and a light sub-unit; they are formed of ribosomal RNA and protein. They consist of three sites which are named: Aminocyl (A- this is where the reading of the mRNA strand takes place), Peptidyl (P- This site is where the polypeptide chain is assembled from) and Exit (E- site where tRNA exits the ribosome).

Once the formed mRNA exits the nucleus, it enters the cytoplasm, where a ribosome attaches to it and tRNA’s are used to assemble polypeptide chains (Protein). (Each three consecutive base pairs on the mRNA strand are called triplet codes; they form codons, which are used in the coding for the synthesis of a certain amino acid. Due to the fact that there are 64 codon combinations and only 20 amino acids; some amino acids can have more than one codon.). Each transfer RNA (tRNA) transfers a specific amino acid and an anticodon to the site of protein synthesis.

The first step of translation is Initiation, which takes place in the P-site of the ribosome. The small and large ribosome sub-unit bind to the mRNA; the small sub-unit sits at the bottom and the large sub-unit which holds the A, P and E sites sit at the top of the mRNA strand. The small ribosomal sub-unit runs along the strand until it reaches the start codon (AUG is the only start codon) that marks the beginning of the reading frame. This is when tRNA comes along with the anti-codon ‘UAC’, which binds to the start codon through complementary base pairing (the Adenine binds with the Uracil, Uracil binds with Adenine, and Guanine binds with Cytosine).

The elongation step begins as translation of the codons continue by the reading of one codon at a time on the mRNA. The reading is done at the A-site. The tRNA enters the ribosome through the A-site, binds to the codon and shifts into the P-site, where it allows the new amino acid to attach itself to the polypeptide chain that is being assembled. Each time a new tRNA enters through the A-site and shifts into the P-site (once it attaches to the codons on the mRNA strand) as the one in the P-site exits the ribosome through the E-site. As this process carries on, the mRNA moves along the ribosomal sub-units.

The termination step begins when a stop codon (UAA, UAG, UGA) goes in the A-site. This ends the translation process because there are no tRNA’s existing with an anticodon which is complementary to the stop codons.

The polypeptide chain unhooks itself from the tRNA and tRNA exits the ribosome from the E-site. The large and small sub-units of the ribosome, part from the mRNA. mRNA self-destructs after this process, while tRNA carries on its function in another translation process. The formed protein chain is then transferred into the Golgi apparatus (in some cases these modifications can occur in the Endoplasmic Reticulum) for further modification before being secreted.

Polysome formation is when more than one ribosome translates the same mRNA strand at the same time.