DNA Translation, Replication, and Transcription

Terms in this set (69)

Translation is a process that decodes the information of mRNA into the sequence of amino acids that eventually form a protein. This process occurs after transcription. Translation occurs in the 5'-3' direction. Once the mRNA is at the ribosome, translation starts. In both eukaryotes and prokaryotes, translation occurs in 3 stages: initiation, elongation, and termination.
Before initiation can occur, specific tRNA-activating enzymes catalyze the attachment of amino acids to tRNA molecules, using ATP for energy. Then initiation begins and the small ribosomal subunit binds to the 5' end of mRNA and moves along it until it reaches the start codon (AUG). mRNA is used as a template and is read as 3 bases (triplets) at a time. Three bases pairs is called a codon. Each codon specifies which amino acid is added to the growing polypeptide. tRNA has a specific amino acid attached. The appropriate tRNA molecule binds to the codon via its anticodon (according to complementary base pairing). Finally, the large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit.
The next stage is elongation, the second tRNA molecule pairs with the next codon in the ribosomal A-site. The amino acid in the P-site is covalently attached by a peptide bond to the amino acid in the A-site. In translation, a process called translocation occurs. Basically, just the moving from one spot to the other.
The ribosome moves along the mRNA sequence in a 5' - 3' direction, synthesizing a polypeptide chain. Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes). The last stage of translation is termination. Elongation and translocation continue until the ribosome reaches a stop codon. These codons do not code for any amino acids and instead signal for translation to stop. The polypeptide is released and the ribosome disassembles back into subunits.
Transcription is the first process by which the DNA code is used to build polypeptides. Transcription transfers sections of the genetic code from DNA to an mRNA molecule. Transcription happens in the nucleus and occurs in the 5'-3' direction.
At the start of transcription, the DNA molecule is separated into two strands by the enzyme RNA polymerase, which binds to the DNA near the beginning of a gene. Hydrogen bonds between the bases are broken and the double helix unwinds.
Transcription begins at a specific point on the DNA molecule called the promoter region, which is a short sequence of non-coding DNA just before the start of the gene. Only one of the strands is used a template for transcription which is called the antisense strand. The other DNA strand is called the sense strand. RNA polymerase uses free nucleoside triphosphate (NTPs) to build the RNA molecule, using complementary base pairing to the DNA and condensation reactions between the nucleotides. This produces a primary mRNA molecule that is complementary to the antisense strand being transcribed, and has the same base sequence as the sense strand (except it has the base Uracil instead of Thymine). RNA polymerase moves along the antisense strand in a 3' to 5' direction. As it does so, the 5' end of a nucleotide is added to the 3' end of the mRNA molecule so that the construction of the mRNA proceeds in a 5' to 3' direction. Eventually, the RNA polymerase reaches the terminator region, which indicates the end of the gene and stops the RNA polymerase and ultimately transcription. The RNA polymerase releases the completed mRNA and finishes rewinding the DNA before breaking free.
Explain the process of DNA replication. [8]
DNA replication is semi conservative, occurs in the 5'-3' direction and occurs during the S phase of mitosis. The first stage of DNA replication is the uncoiling of the DNA double helix by the enzyme helicase. Helicase separates the DNA into two template strands by breaking the hydrogen bonds between base pairs. RNA primase then adds a short sequence of RNA to the template strands. This short sequence of RNA is a primer which allows DNA polymerase III to bind to the strands and start the replication process. Once this is done, DNA polymerase III adds deoxynucleoside triphosphates (dNTPs) to the 3' end of the primer and then to the forming strand of DNA. Deoxynucleoside triphosphates contain the base, sugar and three phosphates and are the precursors to a nucleotide. Two of the three phosphates groups break off during the replication process to release energy. The dNTPs pair up opposite their complementary base partner (adenine pairs with thymine ; guanine pairs with cytosine). Since the strands are anti-parallel (the two strands have their 5' end and 3' end on opposite ends) and the DNA opens in one direction one strand can be replicated continuously (leading strand- 3') but the other must be built is small sections (lagging strand- 5'). These fragments are called Okazaki fragments. DNA polymerase I will remove the RNA primers and replace these with DNA. The enzyme DNA ligase then joins the Okazaki fragments together to form a continuous new strand of DNA.