Protein Synthesis

Transcription

Transcription

Initiation

Elongation

Termination

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

5’ end of mRNA transcript

7-methyl guanosine forming a modified guanine nucleoside triphosphate

Part 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 transcriptPrimary transcript gets capped

Primary transcript gets tailed

The introns get excised

Primary transcript transformed to mRNA transcript

mRNA transcript exits nucleus into the cytoplasm

mRNA transcript ready to be translated by a ribosome into a protein

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