Protein Synthesis Flashcards
Intro
The amino acid sequence of a protein is coded for by triplets (codons) in the sequence of bases along a strand of a DNA molecule. A section of DNA that contains the complete sequence of codons to code for an entire protein is called a gene. Information in a gene is ‘transcribed’ into RNA molecules and then ‘translated’ into a specific amino acid sequence.
Why the base sequence of genes have to be copied and transported to the site of protein synthesis, a ribosome
In a eukaryotic cell, DNA is contained within a double membrane called the nuclear envelope that encloses the nucleus. This protects DNA from being damaged in the cytoplasm. Protein synthesis occurs in the cytoplasm at ribosomes, but a chromosomal DNA molecule is too large to leave the nucleus to supply the coding information needed to determine the proteins amino acid sequence.
To get around this problem the base sequence of genes have to be copied and transported to the site of protein synthesis, a ribosome. This process is called transcription and produces shorter molecules of RNA.
Similarities of transcription with DNA replication
The section of DNA that contains the gene unwinds and unzips under the control of a DNA helicase, beginning at a start codon. This involves the breaking of hydrogen bonds between the bases.
Two types of strands in transcription
Only one of the two strands of DNA contains the code for the protein to be synthesised. This is the sense strand and it runs from 5’ to 3’. The other strand (3’ to 5’) is a complementary copy of the sense strand and doesn’t code for a protein. This is the antisense strand and it acts as the template strand during transcription, so that the complementary RNA strand formed carries the same base sequence as the sense strand.
What do free RNA nucleotides do in transcription
Free RNA nucleotides will base pair with complementary bases exposed on the antisense strand when the DNA unzips. The thymine base in RNA nucleotides is replaced with the base uracil (U). So RNA uracil bonds to adenine on the DNA template strand.
Final steps of transcription
Phosphodiester bonds are formed between the RNA nucleotides by the enzyme RNA polymerase. Transcription stops at the end of the gene and the completed short strand of RNA is called messenger (m)RNA. It has the same base sequence as the sequence of bases making up the gene on the DNA, except that it has uracil in place of thymine.
The mRNA then detaches from the DNA template and leaves the nucleus through a nuclear pore. The DNA double helix reforms. This mRNA molecule then travels to a ribosome in the cell cytoplasm for the next step in protein synthesis.
What is ribosomal (r)RNA
In eukaryotic cells, ribosomes are made up of two subunits, one large and one small. These subunits are composed of almost equal amounts of protein and a form of RNA known as ribosomal (r) RNA. rRNA is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.
What is translation
After leaving the nucleus, the mRNA binds to a specific site in the small subunit of a ribosome. The ribosome holds mRNA in position while it is decoded, or translated, into a sequence of amino acids. This process is called translation.
What is Transfer (t)RNA
Another form of RNA, which is necessary for the translation of the mRNA. It is composed of a strand of RNA folded in such a way that three bases, called the anticodon, are at one end of the molecule. This anticodon will bind to a complementary codon on mRNA following the normal base pairing rules. The tRNA molecules carry an amino acid corresponding to that codon.
When the tRNA anticodons bind to complementary codons along the mRNA, the amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA.
This cannot happen all at once. Instead amino acids are added one at a time and the polypeptide chain (protein) grows as this happens. Ribosomes act as the binding site for mRNA and tRNA and catalyse the assemble of the protein.
Stages of translation
- The mRNA binds to the small subunit of the ribosome at its start codon (AUG).
2.A tRNA with the complementary anticodon (UAC) binds to the mRNA start codon. This tRNA carries the amino acid methionine.
3.Another tRNA with the anticodon UGC and carrying the corresponding amino acid, threonine, then binds to the next codon on the mRNA (ACG). A maximum of two tRNAs can be bound at the same time.
4.The first amino acid, methionine, is transferred to the amino acid (threonine) on the second tRNA by the formation of a peptide bond. This is catalysed by the enzyme peptidyl transferase, which is an rRNA component of the ribosome.
5.The ribosome then moves along the mRNA, releasing the first tRNA . The second tRNA becomes the first.
Stages 3-5 are repeated, with another amino acid added to the chain each time. The process keeps repeating until the ribosome reaches the end of the mRNA at a stop codon and the polypeptide is released.
As the amino acids are joined together forming the primary structure of the protein, they fold into secondary and tertiary structures. This folding and the bonds that are formed are determined by the sequence of amino acids in the primary structure. The protein may undergo further modifications at the Golgi apparatus before it is fully functional and ready to carry out the specific role for which it has been synthesised.
Many ribosomes can follow on the mRNA behind the first, so that multiple identical polypeptides can be synthesised simultaneously.
