SL PROTEIN SYNTHESIS Flashcards
(12 cards)
Transcription as the synthesis of RNA using a DNA template
- WHAT IS TRANSCRIPTION?
- occurs in 5’ to 3’ direction (of mRNA)
- starts at start codon and stops at stop codon
Students should understand the roles of RNA polymerase in this process.
note: RNA molecule is released!
- Transcription is the synthesis of RNA, using DNA as a template. It takes place in the nucleus. The cell`s machinery copies the gene sequence into messenger RNA (mRNA) a molecule that is similar to DNA. The manufactured RNA is single stranded.
- binds to a site on DNA at the start of the gene being transcribed
- unwinds the DNA doubles helix & separates strands (anti-sense/sense or template/coding)
- moves along DNA strand
- attaches RNA nucleotides where hydrogen bonds can be formed
- covalent bonds between sugar and phosphate are formed
- detaches RNA strand from template strand and allows double helix to reform
3 main stages of protein synthesis
- initiation (start codon)
- elongation
- termination (stop codon)
Role of hydrogen bonding & complementary base pairing in transcription
- what does it ensure?
Include the pairing of adenine (A) on the DNA template strand with uracil (U) on the RNA strand.
ensures genetic code in DNA is accurately transcribed to mRNA
Stability of DNA templates
Single DNA strands are used as a template for transcribing a base sequence, without the DNA base sequence changing.
In somatic cells that do not divide, such sequences must be conserved throughout the life of a cell.
can be compromised by what?
The fact that DNA is stable and doesn’t change its code easily is important for the conservation of the original code. The stability is ensured by the sugar-phosphate backbone and hydrogen bonds between nucleotides.
For some somatic (body) cells which don’t divide to replace itself – e.g nerve cells – this means the code must stay unchanged throuhout a lifetime.
The stability of DNA may become compromised by free radicals, chemicals, cigarette smoke or exposure to UV or nuclear radiation. This damage can lead to a (harmful or beneficial) mutation.
Transcription as a process required for the expression of genes
Limit to understanding that not all genes in a cell are expressed at any given time and that transcription, being the first stage of gene expression, is a key stage at which expression of a gene can be switched on and off.
- Gene expression is the process by which information carried by a gene is turned into an observable effect on an organism.
- The DNA sequence itself does not determine the observable characteristics, only the specific sequence which is transcribed does.
- example: insulin is only expressed in pancreatic B cells; in all other cells this gene is never normally transcribed
Translation as the synthesis of polypeptides from mRNA
- what is translation?
- what is a codon?
- what determines polypeptide shape?
The base sequence of mRNA is translated into the amino acid sequence of a polypeptide.
- Translation is the synthesis of a polypeptide or protein from the base sequence of the mRNA.
- Three nucleotide bases of the mRNA code for one amino acid.
-The sequence of amino acids determines the shape of the polypeptide and therefore the protein
Roles of mRNA, ribosomes and tRNA in translation
Students should know that mRNA binds to the small subunit of the ribosome and that two tRNAs can bind simultaneously to the large subunit.
- Transfer RNA (tRNA) molecules translate the base sequence of mRNA to an amino acid sequence. They have an anticodon of three bases that bind to a codon on mRNA via complementary base pairing. tRNA molecules carry amino acids corresponding to their codon
- The ribosomes act as the binding site for mRNA and tRNA. They catalyse the peptide bonds between amino acids of the polypeptide.
- mRNA has a site to which a ribosome can bind and a sequence of codons that specifies the amino acid sequence of the polypeptide (including start and stop codon).
Complementary base pairing between tRNA and mRNA
Include the terms “codon” and “anticodon”.
Features of the genetic code
- Students should understand the reasons for a triplet code.
- Students should use and understand the terms “degeneracy” and “universality”.
The genetic code is said to be universal. All organism use the same 4 letter code (C, T, A and G).
Different codons can code for the same amino acid – this is referred to as degenerate.
Using the genetic code expressed as a table of mRNA codons
Students should be able to deduce the sequence of amino acids coded by an mRNA strand.
Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain
Focus on elongation of the polypeptide, rather than on the special events of initiation and termination.
- Small ribosomal unit is attached to start codon of mRNA sequence
- First tRNA molecule attaches to ribosome at P site w/ anticodon attached to corresponding codon on mRNA sequence
- large ribosmal unit binds to close complex
- three sites - e, p, a
- second tRNA molecule with the corresponding anticodon attaches to codon on mRNA sequence at A site
- PEPTIDE Bonds form between amino acids both sites - polypeptide formed moved to A site
- translation continues in 5’ to 3’ direction, but tRNA molecules shift left in positions
- empty tRNA molecule at E site is released
- elongation occurs until stop codon is reached
Mutations that change protein structure
Include an example of a point mutation/single base substitution affecting protein structure - SICKLE CELL
consequences: body can’t deliver oxygen/nutrients effectively, short red blood cell life span, pain in organs
A gene mutation is a change to the base sequence of a gene.
- Sickle cell anemia is a disease which is caused by a mutation in the DNA which changes the oxygen transporting polypeptide structure of the protein hemoglobin which is contained in red blood cells. (mutation in chromosome 11)
- sickle cell hemoglobin proteins aggregate and form fibers, causing red blood cells to become rigid and sickle-shaped
- The normal biconcave disc shape of the red blood cell is changed to a ‘sickle’ shape.
