5:Protein synthesis - Transcription + translation Flashcards
What is transcription?
Transcription involves DNA being copied into a complementary strand of RNA eg. mRNA, tRNA, rRNA.
What are genes, how are they organised and what are the elements within them?
Genes are the basic physical and functional units of heredity. They are composed of DNA sequences that carry the information required for the construction of proteins. A human cell contains a nucleus that consisting of 30,000 - 40,000 genes. A complete set of genes is called a genome, these are organised into chromosomes. The number of chromosomes within an organism is dependent on the species.
Genes also consist of exons, introns and control elements.
- Exons: The protein coding sections of the gene. ‘Expressed sequences’
- Introns: The non-protein coding sections of the gene ‘Intervening sequences’
There are also control elements for DNA transcription and translation.
Explain the process of transcription
1) Initiation - RNA polymerase binds to the promoter with the help of transcription factors. It unwinds and separates the two DNA strands and initiates mRNA synthesis at the initiation site of the template DNA strand.
2) Elongation - As the RNA polymerase moves along the template strand, the transcript being formed is being elongated. RNA polymerase rewinds the double helix behind it after it has been transcribed
3) Termination - When the RNA polymerase reaches the termination site, transcription stops and the RNA polymerase and the new RNA transcript are released.
Explain 3 mRNA processes that occur in the nucleus; Capping, Splicing & polyadenylation
Capping - The addition of a modified nucleotide at the 5’ end of the strand during transcription to protect the mRNA strand from being degraded by ribonucleases.
Splicing - The removal of introns (intervening sequences) to increase the coding potential resulting in several different products from the same gene that carry out different functions and are expressed in different locations.
Polyadenylation - The addition of an adenine (A) tail at the 3’ end of the mRNA; Also known as the Poly-A tail. It ensures the stability of the mRNA strand.
How is protein expression controlled?
DNA transcription is the primary mechanism for controlling the level of expression of proteins within cells.
Explain what is meant by the genetic code
A sequence of 3 nitrogenous bases (also known as a triplet or a codon) specifies a single amino acid, meaning that coding information must be read in groups of three. More than one codon can represent the same amino acid. 64 different codons, but only 20 amino acis.
The start codon (AUG) gives the signal for the initiation of translation.
The stop codon (UAA, UAG, UGA) gives the signal for the termination of translation.
What are the 3 RNA requirements for protein synthesis?
Protein synthesis requires 3 types of RNA. mRNA attaches to the ribosome and binds the tRNA via its complementary anticodon. rRNA + the protein = ribosome complex.
What is the ribosome?
The ribosome is the site of translation and consists of a binding site for mRNA and 3 binding sites for tRNA (E,P,A). The ribosome is made up of two subunits (The small and large) as well as rRNA.
What is the tRNA anticodon?
The tRNA anticodon is a tight loop located halfway along the tRNA molecule, consisting of 3 nitrogenous bases that bind to the complementary mRNA codon. It is responsible for specifying what amino acid that tRNA will bind to. tRNA binds to the amino acid and carries it to the ribosome.
What is translation?
Translation is the process by which mRNA is translated into proteins. During transcription the template strand runs anti-parallel (and complementary) to the coding strand that undergoes transcription. In the produced RNA, thymine bases are replaced the a uracil base.
What are the key steps associated with translation and explain what is involved in each.
1) Initiation - mRNA binds to the 40S subunit of the ribosome. The ribosome then locates the start codon (AUG). The initiator tRNA binds to the ribosome along with its bound amino acid (F-Met). The 60S subunit then binds to complete the initiation complex.
2) Elongation - Aminoacyl-tRNA (tRNA carrying an amino acid) enters the A binding site causing its amino acid (F-Met) to attach to Valine, releasing it from the tRNA. tRNA is then translocated by the ribosome to the E binding site, where it is released.
3) Termination - The ribosome complex reaches the termination codon (UAA, UAG, UGA), causing release factors to bind. These factors aid in the release of the polypeptide chain. The ribosome-mRNA complex is then dismantled.
