BIOL #13: Protein Synthesis Flashcards
Gene Expression
the process by which DNA directs the synthesis of proteins or RNA molecules
Genotype and Phenotype
All expressed traits (eye color, hair type, etc.) are determined by the synthesis of particular proteins and RNA molecules involved in protein synthesis
- Proteins are the link between genotype (genetic makeup) and phenotype (outward expression of genes)
+ Cells synthesize and breakdown most biological molecules via metabolic pathways. These pathways include a number of enzymes (proteins) and such pathways may result in the synthesis of important molecules (e.g. eye color is a trait that results from the synthesis of pigment in cells of the iris).
Protein Synthesis
The expression of genes that code for proteins involves two main processes:
- Transcription
- Translation
Protein synthesis can result from external signals (pictured here) or internal cellular signals
RNA plays a vital role in protein synthesis:
- RNA contains a ribose sugar rather than a deoxyribose sugar
- RNA contains the bases A, G, C, U (uracil)
- RNA is typically single stranded
Transcription is synthesis of RNA using information in the DNA
Translation is the synthesis of a polypeptide using the information from the transcribed RNA
Prokaryotes vs Eukaryotes
Transcription and translation occurs in all organisms
The basic mechanisms for transcription and translation are similar for bacteria and eukaryotes with one major difference:
- The lack of a nuclear membrane in prokaryotic cells means that transcription and translation is not separated in space and time in bacterial cells – translation of mRNA may begin while it is still being transcribed
Change in Language from DNA to protein
For DNA or RNA (polymers), the monomers are the four types of nucleotides, which differ in nitrogenous bases
For proteins/polypeptides (polymers), the monomers are amino acids
In protein synthesis – DNA monomers must be transcribed then translated into protein monomers
There are 20 different amino acids
How are four DNA bases translated into 20 different amino acids?
The instructions for a polypeptide are written in the DNA as a series of non-overlapping, three-nucleotide words (triplets), called codons.
Transcription
The first step in converting genetic information into proteins is transcription – the synthesis of an RNA version of the instructions stored in DNA.
The DNA strand to be transcribed is called the template strand
The RNA strand built to be complementary to the template strand is called messenger RNA (mRNA)
- Note: mRNA uses uracil (U) in place of thymine (T)
Transcription involves three stages:
1) Initiation
2) Elongation
3) Termination
Initiation (Transcription)
Initiation is the first stage of transcription:
- RNA polymerase begins transcription by prying the two strands of DNA apart and synthesizing the mRNA strand based on the template strand
While RNA polymerases do not require primers to begin transcription, they cannot initiate transcription on their own.
RNA polymerases must attach to promoters to begin transcription
- Promoters are specific DNA sequences that come before the start point of transcription (i.e. upstream of the transcription site)
- RNA polymerase binds in a precise location and orientation on the promoter, which determines where transcription starts and which of the two strands of the DNA helix is used as the template
RNA Polymerase
Like the DNA polymerases, an RNA polymerase can only perform template-directed synthesis in the 5′ to 3′ direction.
Unlike DNA polymerases, RNA polymerases do not require a primer to begin synthesis.
Bacteria have one RNA polymerase while eukaryotes have three distinct types, RNA polymerase I, II, and III.
Initiation & Promoters: Bacteria vs Eukaryotes
In bacteria, the RNA polymerase itself can recognize and bind to the promoter region
In eukaryotes, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription
- Many of the eukaryotic promoters include a unique sequence called the TATA box, centered about 30 base pairs upstream of the transcription start site.
- Only after transcription factors are attached to the promoter can RNA polymerase II bind to the promoter
- The transcription factors plus RNA polymerase bound to the promoter is called the transcription initiation complex
Elongation (Transcription)
During the elongation phase of transcription, RNA polymerase moves along the DNA template and synthesizes RNA in the 5’ 3’ direction.
Termination (Transcription)
The mechanisms of termination differ between bacteria and eukaryotes:
In bacteria, there is a terminator sequence in the DNA that causes the RNA polymerase to detach from the DNA and release the mRNA transcript.
In eukaryotes, RNA polymerase transcribes a sequence on the DNA called the polyadenylation signal sequence, which codes for a polyadenylation signal (AAUAAA) in the mRNA transcript. At a point ~10-35 nucleotides downstream from the AAUAAA signal, proteins associated with the growing mRNA transcript cut and release the transcript.
RNA Processing
In bacteria, the information in DNA is converted directly to mRNA.
In eukaryotes, the product of transcription is an immature primary transcript, or pre-mRNA. Before primary transcripts can be translated, they have to be processed in a complex series of steps.
- Specific enzymes are involved in the RNA processing that produces a mature mRNA:
+ Both ends of the pre-mRNA are altered
+ Certain interior sections of the pre-mRNA are cut out and the remaining sections are spliced together
Exons vs. Introns
The protein-coding regions of eukaryotic genes are interrupted by noncoding regions.
- To make a functional mRNA, these noncoding regions must be removed.
Exons are the coding regions of eukaryotic genes that will be part of the final mRNA product.
The intervening noncoding sequences are called introns, and are not in the final mRNA.
RNA Splicing
The transcription of eukaryotic genes by RNA polymerase generates a primary RNA transcript (pre-mRNA) that contains exons and introns.
- Introns are removed by splicing.
Short nucleotide sequences at each end of an intron signal splicing sites
Small nuclear ribonucleoproteins (snRNPs) recognize these splice sites.
snRNPs plus proteins form a complex called a spliceosome, which catalyzes the splicing reaction.
Alternative RNA Splicing
Some introns might contain sequences that regulate gene expression or affect gene products.
- The prevalence of such functions are still under debate
An important consequence of the presence of introns in genes is that a single gene can encode more than one kind of polypeptide:
- Many genes can give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing – this is called alternative RNA splicing
+ Example: sex differences in fruit flies are mostly due to differences in how males and females splice the RNA transcribed from particular genes
