Protein Synthesis

Protein Synthesis Transcription

Transcription

Transcription- Initiation • The enzyme, RNA Polymerase, binds to the segment of DNA that is to be transcribed • Once, RNA Polymerase binds, it opens the double helix and transcription begins • This occurs both in prokaryotes and eukaryotes • DNA transcription is not a random phenomenon • If RNA polymerase randomly transcribed DNA, the cell would not make the correct polypeptide

Transcription- Initiation • RNA polymerase binds to the DNA molecule upstream of the gene to be transcribed • Upstream: Region of DNA adjacent to the start of a gene • The upstream region is a sequence on one strand of DNA located adjacent to the start of the gene

Upstream Region • This region indicates where the RNA polymerase should start transcribing and which DNA strand should be transcribed • This upstream region is know as the promoter • In most genes, the promoter consists of a characteristic base-pair pattern

Promoter • Promoter region in DNA is high in adenine and thymine bases, which serve as a recognition site for RNA Polymerase • A=T share only 2 hydrogen bonds as compared with C and G that share 3 H bonds • It takes less energy to break to 2 bonds instead of 3

Transcription-Elongation • Elongation starts as soon as the RNA Polymerase moves to the start of the gene and binds to the promoter • It then opens up the double helix and starts building the single-stranded mRNA in the 5'-3' direction • Unlike the case in DNA replication, the RNA polymerase does not require a primer to start building the complementary strand

Transcription- Elongation • Note: The promoter itself does not get transcribed • The process of elongation of the RNA transcript is similar to that of DNA replication • RNA uses only one of the strands of DNA as a template for mRNA synthesis • This chosen DNA strand is called the template strand • The strand not used for transcription is the coding strand

Know your strands • DNA Replication • Done by DNA polymerase • DNA synthesized in the 5'-3' direction • Leading strand: strand synthesized continuously, using the 3'-5' template strand as its guide and is build toward the replication fork • Lagging strand: strand synthesized discontinuously in the opposite direction to replication fork • mRNA Transcription • Done by RNA polymerase • mRNA synthesized in the 5'-3'direction • Template strand: strand of DNA that RNA polymerase uses as a guide to build complementary mRNA • Coding strand: strand of DNA that is not used for transcription and is identical in sequence to mRNA except it contains thymine instead of uracil* (p.242)

Know your strands Both DNA and mRNA are synthesized in the 5'-3' direction

Transcription- Termination • mRNA is synthesized until the end of the gene is reached • Terminator sequence: RNA Polymerase recognizes the end of the gene when it comes across this sequence • Then, the newly synthesized mRNA dissociates with the DNA template strand • Transcription stops and RNA Polymerase is free to bind to another promoter region and transcribe another gene

mRNA Synthesis

Posttranscriptional Modifications: • In eukaryotes, mRNA cannot leave the nucleus directly following transcription • mRNA must be modified to primary transcript • A 5' cap is added to the start of the primary transcript

Structure of the 5' cap 7-methyl guanosine forming a modified guanine nucleoside triphosphate Part of mRNA transcript 5’ end of mRNA transcript

5' cap • Added to the start of primary transcript • Function: protects the mRNA from digestion by enzymes such as nucleases and phosphatases as it exits the nucleus and enters cell cytoplasm • Plays a role in initiation of translation

Further mRNA Modificatin: • Enzyme poly-A polymerase involved in furhter modification of mRNA • Enzyme responsible for adding a string of ~ 200 adenine ribonucleotides to the 3' end of mRNA • This string is known as the poly-A tail • This is done to protect the mRNA from degradation later on Poly-A tail

Capping and Tailing • Addition of a 5' cap and a poly-A tail to the primary transcript is a process known as capping and tailing

Further Modification • DNA of a eukaryotic gene is made of various regions • Exons: Segments of DNA that code for part of a specific protein • Introns: Non-coding regions of a gene • The introns are interspersed among the exons

Introns and Exons • Primary transcript contains both introns and exons, i.e, it contains both regions that code and those that do not code for part of the protein • If the introns (non-coding regions) are translated, the protein will not fold properly causing the proteins to be dysfunctional and useless to the cell • Therefore, before the primary transcript leaves the nucleus, the introns are removed

Spliceosomes • Particles made of RNA and protein • They cut introns from mRNA primary transcript • And join together the remaining coding exon regions • mRNA exits the nucleus, but the introns stay, where they get degraded and recycled

mRNA transcript

mRNA is not error proof • Unlike DNA replication, there is no quality control enzyme to ensure that the mRNA transcript is correct • Therefore, more errors are made during transcription than translation • Errors are not as detrimental as those occurring during DNA replication, since a single gene is transcribed repeatedly to make hundreds of transcripts

Protein Synthesis