4A - DNA, RNA And Protein Synthesis Flashcards
Is the structure of DNA the same in all organisms?
Yes, but eukaryotic and prokaryotic cells store DNA in different ways.
Describe (nuclear) eukaryotic DNA.
- DNA molecules are LONG and LINEAR
- Wound around proteins called HISTONES
- The DNA and proteins are then coiled up tightly to make a compact chromosome
- Chromosomes exist in the nucleus
(See diagram pg 82 of revision guide)
Describe prokaryotic DNA.
- DNA molecules are SHORT and CIRCULAR
- Not wound around histones, but do form chromosomes
- Chromosomes fit in cell by supercoiling
(See diagram pg 82 of revision guide)
Compare nuclear eukaryotic DNA and prokaryotic DNA.
EUKARYOTIC: • Linear • Long • Associated with histones PROKARYOTIC: • Circular • Short • Not associated with histones
What are histones?
Proteins around which nuclear eukaryotic DNA is wound.
What is the purpose of histones?
- Winding of eukaryotic DNA around the histones makes it more compact
- Histones support the DNA
Describe the process by which nuclear eukaryotic DNA is stored.
1) DNA is in a double helix with a linear shape.
2) DNA is wound around histone proteins.
3) DNA with the histones is coiled up repeatedly -> Makes a single chromosome.
What are the two types of DNA in eukaryotic cells?
- Nuclear DNA
* Mitochondrial / Chloroplast DNA
Compare nuclear and mitochondrial/chloroplast DNA in eukaryotes.
- Nuclear -> Linear, long + associated with histones
- Mitochondrial/Chloroplast -> Circular, short + not associated with histones
(i.e. The DNA in mitochondria and chloroplasts is like prokaryotic DNA.)
By what process is prokaryotic DNA made to fit in the cell?
Supercoiling
Describe the process by which prokaryotic DNA is stored.
1) DNA is in a double-helix.
2) The double helix is in a circular chromosome shape.
3) Chromosome is coiled.
4) The DNA is then supercoiled to fit in the cell.
What is a gene?
A sequence of DNA bases that codes for either a polypeptide or functional RNA.
What are the two things a gene can code for?
- Polypeptide
* Functional RNA
What is the primary structure of a protein?
The order of amino acids in its polypeptide chains.
How does the order of DNA bases in a gene code for the polypeptide produced?
- 3 bases in the DNA code for one amino acid in the chain
* The order of bases in a gene determines determines the order of amino acids in a polypeptide
What is a triplet?
The three DNA bases in a gene that code for a single amino acid.
What is functional RNA?
- RNA molecules other than mRNA (!), which perform special tasks during protein synthesis.
- e.g. tRNA and rRNA
Is mRNA a type of functional RNA?
No
Give two examples of functional RNA.
- tRNA
* rRNA
What is a cell’s genome?
The complete set of genes in the cell.
What is a cell’s proteome?
The full range of proteins that the cell is able to produce.
Do all parts of eukaryotic genes code for amino acids?
No, some parts are non-coding (introns).
NOTE: Some genes as a whole don’t code for polypeptides, but functional RNA.
What are introns and exons?
- Introns -> Sections of genes that don’t code for amino acids
- Exons -> Sections of genes that do code for amino acids
How can you remember the difference between introns and exons?
EXons - EXpressed
INtrons - stay IN
Does prokaryotic DNA have introns?
No
What happens to introns in DNA?
- They are removed from pre-mRNA in a process called splicing -> After transcription.
- This leaves only the exons.
Apart from genes, what does a DNA sequence contain?
Non-coding repeats
What are non-coding repeats?
- DNA sequences outside of genes that don’t code for amino acids (similar to introns, but outside of genes)
- e.g. CCTTCCTTCCTT
Describe all the different parts of a DNA sequence.
DNA sequence is split into genes and non-coding repeats.
NON-CODING REPEATS:
• Outside of genes -> Don’t code for amino acids
GENES:
• Genes code for either a polypeptide OR functional RNA
• If they code for a polypeptide, they are split into coding parts (exons) and non-coding parts (introns)
• If they code for functional DNA, they can code for tRNA, rRNA, etc. -> Needed in protein synthesis
What are alleles?
- Different types of the same gene
- Code for slightly different versions of the same polypeptide
- e.g. The allele for blue eyes and the allele for brown eyes
How many chromosomes do humans have?
23 pairs (or 46 in total)
What is a homologous pair?
- A pair of matching chromosomes of the same side and containing the same genes (although they oils have different alleles).
- e.g. The 21st pair of chromosomes in humans
What structure does “homologous pair” refer to?
Chromosomes (not genes or alleles!)
Give an example of a homologous pair.
The 21st pair of chromosomes in humans.
Do homologous pairs have to have the same alleles?
No, but they need to have the same genes and be the same size.
What is the position of a gene on a chromosome called?
The locus.
What can be said about the position of alleles coding for a certain characteristic in a chromosome?
They will be found at the same locus.
Remember to revise the diagram of homologous chromosomes on pg 83.
Do it.
What does uracil pair with in protein synthesis?
Adenine
What is the difference between DNA and RNA?
- DNA is a double-helix, while RNA is a single polynucleotide strand
- DNA contains thymine, while RNA contains uracil as a base
What are the two types of RNA involved in protein synthesis?
- Messenger RNA (mRNA)
* Transfer RNA (tRNA)
What does the m in “mRNA” stand for?
Messenger
What does the t in “tRNA” stand for?
Transfer
What are the two stages of protein synthesis?
- Transcription
* Translation
What is the function of mRNA?
- Made during transcription
* Carries the genetic code from the DNA to the ribosomes, ready for translation
Describe the structure of mRNA.
- Single polynucleotide strand -> Ribose sugar - phosphate backbone with bases attached
- Appears linear
(See diagram pg 84 of revision guide)
What are 3 adjacent bases on mRNA called?
Codons (sometimes triplets or base triplets)
What is the function of tRNA?
- Involved in translation
* Carries amino acids (used to make proteins) to the ribosomes
Describe the structure of tRNA.
- Single polynucleotide strand -> Folded into a clover shape
- Hydrogen bonds between specific base pairs hold the molecule in this shape
- Specific sequence of three bases at one end (anticodon)
- Amino acid binding site at other end
(See diagram pg 84 of revision guide)
What is the specific set of 3 bases at the bottom of a tRNA molecule called?
An anticodon.
Remember to practise drawing out the structure of mRNA and tRNA.
Pg 84 of revision guide.
What holds a tRNA molecule in its clover shape?
Hydrogen bonds between specific bases pairs.
In brief, what is transcription?
The process by which an mRNA copy of a gene is made from DNA, so it can be taken to ribosomes.
Where do transcription and translation happen?
- Transcription -> In the nucleus (except in prokaryotes)
* Translation -> Ribosomes in the cytoplasm
Where does transcription take place in prokaryotes?
In the cytoplasm, because they have no nucleus.
Describe transcription.
1) RNA polymerase attaches to the DNA double-helix at the beginning of a gene.
2) Hydrogen bonds between the two DNA strands break, separating the strands -> DNA uncoils partly -> Bases exposed
3) One strand is used as a template to make an mRNA copy.
4) Free RNA nucleotides move in and pair with exposed complementary bases on the DNA strand -> NOTE: Each nucleotide has only one base on it.
5) RNA polymerase moves along the strand joining the sugar-phosphate backbone to create a pre-mRNA molecule.
6) DNA is separated from the pre-mRNA strand.
7) DNA strands rejoin after RNA polymerase has passed.
8) When RNA polymerase reaches a stop signal, it detaches from the DNA.
Is an mRNA nucleotide a chain?
- No, a NUCLEOTIDE is just a SINGLE structure of ribose, phosphate and 1 vase.
- An mRNA MOLECULE is a log chain of RNA nucleotides.
What enzyme is involved in transcription?
RNA polymerase
Describe the pre-mRNA molecule produced by transcription.
It is complementary copy to the original DNA strand.
What are 3 adjacent bases called on:
• DNA
• mRNA
• tRNA
- DNA - Triplet
- mRNA - Codon
- tRNA - Anticodon
In transcription, what do these bases become: • A • C • G • T
- A -> U
- C -> G
- G -> C
- T -> A
What does RNA polymerase do?
- Unwinds the DNA double-helix + exposes bases
* Joins the free RNA nucleotides to together by their sugar-phosphate backbone
What happens to pre-mRNA after transcription is finished in eukaryotes and prokaryotes?
- Eukaryotes -> It is spliced and then moves out of the nucleus through a nuclear pore.
- Prokaryotes -> None of this.
Compare the products of transcription in eukaryotes and prokaryotes.
- Eukaryotes -> Both introns and exons are copied -> Splicing must happen
- Prokaryotes -> No introns in prokaryotic DNA -> No need to splice
When does splicing happen?
After transcription.
How much DNA codes for a single polypeptide?
A gene.
Does splicing happen in prokaryotes?
No, because the DNA has no introns.
What is the difference between pre-mRNA and mRNA?
- Pre-mRNA has both introns and exons.
* mRNA has only exons.
Where does splicing take place?
In the nucleus, before the mRNA leaves it.
What is splicing?
Removing introns from pre-mRNA to form mRNA that has just exons.
Describe the entire process to remove introns.
- DNA strand has introns and exons
- Pre-mRNA made that has introns and exons (transcription)
- Introns removed (splicing)
- Mature RNA is made that has only exons
In brief, what is translation?
When ribosomes join amino acids together to make a polypeptide chain following the sequence of codons carried by the mRNA.
Describe translation.
1) Ribosome attaches to the first codon of a ribosome and free tRNA molecules move towards it, carrying amino acids. (ATP provides energy for bond between amino acid and tRNA molecule to form.)
2) tRNA molecule with an anticodon that’s complementary to the first codon on the mRNA attaches itself to the mRNA by specific base pairing.
3) A second tRNA attaches itself to the next codon in the same way.
4) The two amino acids carried by the two tRNA molecules are joined by a peptide bond -> Using an enzyme and ATP.
5) This repeats over the next codons.
6) As ribosome moves onto the third codon, joining the second and third amino acids, the first tRNA molecule is released and moves away.
7) This repeats, creating a polypeptide chain, until there’s a stop signal on the mRNA molecule.
8) The polypeptide chain moves away from the ribosome.
9) Several ribosomes can pass over the mRNA in succession.
Describe the corresponding mRNA codon and tRNA anticodon for this DNA triplet: ATC.
- DNA = ATC
- mRNA = UAG
- tRNA = AUC
What allows translation to happen?
A ribosome.
What does a ribosome do in translation?
Join adjacent amino acids carried by tRNA together with peptide bonds.
What is needed for the peptide bonds to be formed between amino acids in translation?
- Enzyme
* ATP
Remember to practise drawing and writing out the transcription and translation process.
Pg 84 and 85 of the revision guide.
What is the genetic code?
The sequence of base triplets (codons) in mRNA which code for specific amino acids.
What are the important characteristics of genetic code?
- Non-overlapping
- Degenerate
- Universal
What does non-overlapping mean in terms of genetic code?
- Each triplet is read in sequence, separate from the triplet before and after it.
- Triplets don’t share their bases.
What does degenerate mean in terms of genetic code?
- There are more possible combinations of triplets than there are amino acids.
- So some amino acids are coded for by more than one triplet.
How many amino acids and possible base triplets are there?
- Amino acids - 20
- Triplets - 60
This means that genetic code is degenerate.
What does universal mean in terms of genetic code?
- The same specific base triplets code for the same amino acids in all living things.
- e.g. UAU codes for tyrosine in all organisms.
Do all base triplets code for an amino acid?
No, some are start or stop signals.
What are start or stop codons?
Triplets in mRNA that are used to tell the cell where to start and stop the production of the protein.
Given a table of mRNA codons with their corresponding amino acid, what questions could you be asked?
- Give the corresponding DNA sequence for each amino acid
- Give the tRNA anticodons from mRNA codons
- Write the amino acid sequence for a section of mRNA
Given these mRNA codons, give the corresponding DNA triplets. • UCU • CUA • UAU • GUG • GCA • CGC
- UCU -> AGA
- CUA -> GAT
- UAU -> ATA
- GUG -> CAC
- GCA -> CGT
- CGC -> GCG
Given these mRNA codons, give the corresponding tRNA anticodons.
- UCU -> AGA
- CUA -> GAU
- UAU -> AUA
- GUG -> CAC
- GCA -> CGU
- CGC -> GCG
Given this table of mRNA codons and corresponding amino acids... • UCU - Serine • CUA - Leucine • UAU - Tyrosine • GUG - Valine • GCA - Alanine • CGC - Arginine ...work out the amino acid sequence for this section of mRNA: CUAGUGCGCUAUUCU
Leucine, Valine, Arginine, Tyrosine, Serine
In an investigation of a drug that prevents protein synthesis, the concentration of proteins and mRNA was measured. The concentration of both proteins and mRNA was decreased compared to a control. Suggest what the drug does.
- Inhibits the production of mRNA (transcription) which in turn stops proteins being produced (translation).
- e.g. Could be an enzyme that digests mRNA
In an investigation of a drug that prevents protein synthesis, the concentration of proteins and mRNA was measured. The concentration of both proteins was decreased compared to a control, but protein concentration was the same. Suggest what the drug does.
- Inhibits translation, but leaves mRNA untouched -> So no protein can be produced
- e.g. Could be something that binds to the ribosome, blocking tRNAs from binding to it, preventing translation.
Remember to practise interpreting data about the role of nucleus acids.
Pg 87 of revision guide
