4.2 DNA and protein synthesis Flashcards
What is a gene?
(a) A sequence of DNA (nucleotides) bases that codes for a polypeptide.
What is a genome?
The complete set of genes present in the cell.
Is the full genome present within every cell?
Yes!
Is every gene expressed in every cell?
No!
What does which genes are expressed depend on?
The cell type
What is the proteome?
The full range of proteins that a cell is able to produce
Is the proteome normally larger than the genome of an organism?
Yes!
Why is the proteome normally larger than the genome?
The large amount of post-translational modification of proteins
Is each gene capable of producing multiple different proteins?
Yes!
Why is each gene capable of producing multiple different proteins?
Alternative splicing
Is RNA a polynucleotide?
Yes
What bases does RNA contain?
A+U and C+G
What sugar does RNA contain?
Ribose
Is RNA double or single stranded?
Single
What type of bonds are the sugar-phosphate bonds in RNA?
Covalent bonds
What do the phosphodiester bonds link in RNA?
- The 5-carbon of one ribose sugar molecule, to the phosphate group of the same nucleotide.
- Which in turn is linked to the 3-carbon of the ribose sugar of the next nucleotide.
Give an example of RNA:
mRNA
Is mRNA single stranded?
Yes
Does mRNA contain a sugar-phosphate backbone?
Yes
mRNA contains exposed _______ bases.
Unpaired
Is tRNA single or double stranded?
Single
Does tRNA have a sugar phosphate backbone?
Yes
What shape does tRNA have?
A folded shape.
How is tRNA a folded shape?
There are hydrogen bonds between some of the complementary bases
Where do amino acids bind to tRNA?
A specific region of the molecule.
What is the specific anticodon found on the tRNA molecule complimentary to?
A specific codon on an mRNA molecule
What are the two stages of protein synthesis?
- Transcription
- Translation
Summary of transcription:
- DNA is transcribed
- An mRNA molecule is produced
Summary of translation?
- mRNA is translated - an amino acid sequence is produced.
Where does transcription occur?
Nucleus
What enzyme catalyses part of the DNA molecule being unwound?
Helicase
How can a DNA molecule be unwound?
By breaking the hydrogen bonds between the complementary base pairs.
What does helicase do (transcription)?
Unwinds part of the DNA molecule
What does unwinding part of the DNA molecule do?
Exposes the gene (that codes for a particular polypeptide) to be transcribed.
How is a complimentary copy of the code from the gene made?
By building a single-stranded nucleic acid molecule known as mRNA.
How is a complementary copy of the gene made?
- Free activated RNA nucleotides pair up.
- With their complementary bases on one strand of the DNA molecule.
How do the free activated RNA nucleotides pair up?
Via hydrogen bonds
What enzyme bonds the sugar-phosphate groups of these RNA nucleotides together?
RNA polymerase
What does RNA polymerase do?
Bonds the sugar-phosphate groups of these RNA nucleotides together
What does RNA polymerase form?
The sugar-phosphate backbone of the mRNA molecule.
What happens once the gene has been transcribed?
- The hydrogen bonds between the mRNA and DNA strands break
- The double stranded DNA molecule re-forms
How does the mRNA molecule leave the nucleus?
Via a pore in the nuclear envelope.
When does the mRNA molecule leave the nucleus?
Once the gene has been transcribed.
Do free activated RNA nucleotides pair up with exposed bases on one or both strands of the DNA molecule?
One
What is the template strand?
The strand of DNA with which free activated RNA nucleotides pair up with its exposed bases.
What is the non-template strand?
The strand of the DNA molecule that is not transcribed
Can the genome in EUKARYOTIC cells contain non-coding section?
Yes!
What are the 2 places the non-coding DNA can be found in the genome in eukaryotic cells?
- Between genes
- Within genes
How is non-coding DNA found BETWEEN genes?
As non-coding multiple repeats
How is non-coding DNA found WITHIN genes?
As introns
When is pre-mRNA produced?
During transcription, when a gene is transcribed (all introns and exons)
What does pre-mRNA contain?
The introns and exons of a certain gene
When does splicing occur?
Before the pre-mRNA exits the nucleus
What happens to the non-coding sections during splicing?
They are removed
What happens to the coding sections during splicing?
They are joined together
What does mRNA contain (after splicing)?
Only exons (the coding sequences)
What is alternative splicing?
- The exons of genes can be spliced in many different ways.
- To produce different mature mRNA molecule.
What does mRNA mean?
A single eukaryotic gene can code for more than one polypeptide chain.
Where does translation occur?
In the cytoplasm
What does mRNA do once it has left the nucleus?
It attaches to a ribosome
Where are tRNA molecules located?
Free, in the cytoplasm
What is on either end of a tRNA molecule?
- On one end there is an anticodon.
- On the other, there is a region where a specific amino acid can attach
How many different tRNA molecules are there?
At least 20
What do tRNA molecules bind with?
Their specific amino acids
What do the tRNA molecules do once they have bound with their specific amino acids?
They bring them to the mRNA molecule on the ribosome
What does the anticodon on each tRNA molecule bind to?
The complementary codon on the mRNA molecule
How many tRNA molecules fit onto a ribosome at one time?
2
Because 2 tRNA molecules can fit on the ribosome at one time, what happens to the amino acids?
2 amino acids are brought side by side
What bond is formed between the 2 side-by-side amino acids?
A peptide bond
What does the formation of a peptide bond require?
Energy, in the form of ATP
When does translation continue until?
A stop codon is reached
What is a triplet?
A sequence of three DNA bases that codes for a specific amino acid
What is a codon?
Sequence of three mRNA bases that codes for a specific amino acid
What is an anticodon?
A sequence of three tRNA bases that are complementary to a codon
Is the genetic code degenerate, and what does it mean?
Yes - multiple mRNA codons can code for the same amino acid
What does the fact that the genetic code is degenerate mean?
A change in the genetic code doesn’t necessarily result in a change in the amino acid sequence
What does the start codon do?
- Marks the start of the protein
- Therefore initiates the process of translation (from the right location).
What does the stop codon do?
Causes translation to terminate at the end of the protein
Does the stop codon code for any amino acids?
NO
Is the genetic code non-overlapping, and what does this mean?
Yes - each base is only read once in the codon it is part of.
Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule.
- tRNA is ‘clover leaf shape’, mRNA is linear
- tRNA has hydrogen bonds, mRNA does not
Describe and explain a difference in the structure of pre-mRNA and mRNA.
- mRNA has no introns/has (only) exons
- Due to splicing
Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation.
- tRNA brings specific amino acid to ribosome
- Anticodon on tRNA binds to codon on mRNA
- Amino acids join by condensation reaction
Describe how mRNA is produced from an exposed template strand of DNA.
- Free RNA nucleotides form complementary base pairs
- Phosphodiester bonds form
- Catalysed by RNA polymerase
Define the term exon.
Base sequence coding for a polypeptide
Describe how mRNA is formed by transcription in eukaryotes. (LONG)
- Hydrogen bonds between DNA bases break
- One DNA strand acts as a template
- Free RNA nucleotides align by complementary base pairing
- Uracil base pairs with adenine
- RNA polymerase joins adjacent RNA nucleotides
- By phosphodiester bonds
Describe how a polypeptide is formed by translation of mRNA.
- mRNA attaches to ribosomes
- tRNA anticodons bind to complementary mRNA codons
- tRNA brings a specific amino acid
- Amino acids join by peptide bonds (using ATP)
- tRNA released after amino acid joined to polypeptide
- The ribosome moves along the mRNA to form the polypeptide
What is the proteome of a cell?
The full range of different proteins that a cell is able to produce
Describe the process of transcription (8 steps).
DNA helicase unwinds DNA double helix
by breaking the hydrogen bonds between the base pairs and strand seperates
Only one strand acts as a template.
Free RNA nucleotides align next to their complementary bases on the template strand and form temporary hydrogen bonds (uracil replaces thymine).
RNA polymerase joins the adjacent nucleotides together by forming phosphodiester bonds in a condensation reaction.
RNA polymerase reaches stop codon, mRNA (prokaryotes) or pre-mRNA (eukaryotes) detaches from DNA.
This forms pre-mRNA in eukaryotic cells and mRNA in prokaryotic cells.
Then the pre-mRNA is spliced (introns removed) to form mRNA.
Describe the process of translation (8 steps).
- mRNA attaches to ribosomes after leaving nucleus through cytoplasm
- tRNA anticodons bind to complementary mRNA codons
- tRNA brings a specific amino acid
- Amino acids join by peptide bonds between 2 amino acids- requires ATP
- Amino acids join together with the use of ATP - by condensation reaction
- tRNA released after amino acid joined to polypeptide
- continue until stop codon
- The ribosome moves along the mRNA to form the polypeptide
Mutation 1 leads to the production of a non-functional protein. Explain why. (3)
(Mutation) changes triplets
Changes amino acid sequence
Affects hydrogen / ionic / sulfur bond
Changes tertiary structure of protein
give the two types of ,molecules forms which a ribosomes is made from
rRNA and protein
give 4 structural differences between a DNA molecule and an MRNA molecule
DNA has deoxyribose, mRNA has ribose;
DNA has thymine, mRNA has uracil;
DNA long, mRNA short;
DNA is double-stranded, mRNA is single-stranded
Accept ‘double helix’ for ‘double stranded’ and ‘single helix’ for ‘single stranded’
DNA has hydrogen bonds, mRNA has no hydrogen bonds
OR
DNA has (complementary) base pairing, mRNA does not;
name 2 biological molecules that can be coded for by a gene, dont include a polypeptide or a protein
pre mRNA
rRNA
tRNA
(c) Scientists investigated the structure of the endoplasmic reticulum.
The table below shows some of the scientists’ results.
Type of endoplasmic reticulum Percentage of endoplasmic reticulum made of phospholipids
Rough 46.8
Smooth 52.5
Use the data in the table to suggest how the structure of rough endoplasmic reticulum is different from the structure of smooth endoplasmic reticulum and how this is related to their functions.
Less phospholipids in rough OR More protein/glycoprotein in rough OR Presence of ribosomes in rough;
Rough ER - production/transport of proteins;
Smooth ER - production/modification/packaging/transport of lipids;
(a) Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation.
a) 1. tRNA brings specific amino acid (to ribosome);
2. Anticodon (on tRNA) binds to codon (on mRNA);
3. Amino acids join by condensation reaction (using ATP).
(b) Describe how the production of messenger RNA (mRNA) in a eukaryote cell is different from the production of mRNA in a prokaryote cell.
______________________
- Pre-mRNA (only) produced in eukaryote;
- Splicing only occurs in eukaryote;
- Introns removed in eukaryote OR introns not present in prokaryote.
(c) Scientists produced a short, single-stranded, artificial nucleic acid, called PNA. The PNA binds to a small section of DNA.
The scientists introduced PNA into cells and discovered that these cells produced less mRNA than cells that did not contain PNA.
Suggest how PNA affected the transcription of the section of DNA.
- PNA is complementary to DNA;
- Preventing/reducing RNA polymerase activity/binding.
(d) 1. Releases/provides energy; - Peptide bonds form between amino acids OR amino acid joins to tRNA.
(d) Describe the role of ATP in the process of translation in protein synthesis.
- Releases/provides energy;
- Peptide bonds form between amino acids OR amino acid joins to tRNA.
(c) Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule.
- tRNA is ‘clover leaf shape’, mRNA is linear;
- tRNA has hydrogen bonds, mRNA does not;
- tRNA has an amino acid binding site, mRNA does not;
- tRNA has anticodon, mRNA has codon.
(d) In a eukaryotic cell, the structure of the mRNA used in translation is different from the structure of the pre-mRNA produced by transcription.
Describe and explain a difference in the structure of these mRNA molecule
d) 1. mRNA fewer nucleotides OR Pre-mRNA more nucleotides;
2. Because of splicing.
(a) Describe how mRNA is produced from an exposed template strand of DNA.
Do not include DNA helicase or splicing in your answer.
- Free RNA nucleotides form complementary base pairs;
- Phosphodiester bonds form;
- By action of RNA polymerase
