8. DNA, GENES, PROTEIN SYNTHESIS Flashcards
Describe what a gene’s ‘loci’ is
A specific place on a chromosome where the gene is located.
What are introns and exons?
Introns are non-coding DNA; exons are coding DNA.
Apart from introns, Where else on a chromosome would you find non-coding DNA?
In between genes
Define a gene
A short section of DNA that codes for amino acids that join to form polypeptide chains
Name the three types of RNA
mRNA, tRNA, rRNA
Recall the four things genes code for
mRNA, tRNA, rRNA, polypeptides
The DNA in eukaryotic cells is universal. What does this mean?
The DNA of living organisms all contain the same bases.
The DNA in eukaryotic cells is universal. How is this indirect evidence for evolution?
All living organisms are made of the same DNA which shows they all have a common ancestor.
The DNA in eukaryotic cells is degenerate. What does this mean?
More than one codon can code for the same amino acid.
The DNA in eukaryotic cells is non-overlapping. What does this mean?
The same base is not used for two different codons (DNA is read in codons that do not overlap).
What is the purpose of transcription?
The production of mRNA from DNA
Where does transcription occur?
Nucleus
Why does transcription in prokaryotes not involve pre-mRNA?
It does not contain introns.
In a DNA molecule, which strand contains the code for the polypeptide?
Sense strand
In transcription, which DNA strand is used as a template to make mRNA?
Antisense strand
Name the two enzymes involved in transcription
DNA helicase and RNA polymerase
What is the function of DNA Helicase?
Unzips DNA by catalysing the break-down of hydrogen bonds between complementary DNA base pairs.
How much of the DNA is unzipped in transcription? Where is it unzipped?
Only the section of the DNA at the gene locus is unzipped.
What is a codon?
Three bases that code for an amino acid.
How many bases are required to code for a polypeptide containing 103 amino acids?
103 x 3 = 309
There are 64 different possible codons the code for amino acids. Show and explain how this is calculated?
There are 4 bases, and each codon has 3 bases. The bases could all be the same so 4 x 4 x 4 = 64
Explain why there are only 20 amino acids, but 64 possible codons
Multiple codons can code for the same amino acid.
From this strand of DNA, write a complementary DNA sequence: TAC-TTT-TCA-AAG-GAT
ATG-AAA-AGT-TTC-CTA
From this strand of DNA, write a complementary mRNA sequence: TAC-TTT-TCA-AAG-GAT
AUG-AAA-AGU-UUC-CUA
How is the start and end of the gene signalled?
By a start and stop codon.
Describe how the complementary mRNA strand is formed
Free RNA nucleotides attach to their exposed complementary base pairs on the antisense strand of DNA through hydrogen bonding
What is the function of RNA Polymerase?
Catalysing the formation of phosphodiester bonds between adjacent RNA nucleotides to produce pre-mRNA
How is mature mRNA formed from pre-mRNA?
Introns are spliced out and exons join to form mature mRNA.
Once mature mRNA is formed from pre-mRNA,, what happens to the mRNA?
mRNA travels out of the nucleus out of a nuclear pore to the cytoplasm.
What is the purpose of translation?
To make a polypeptide chain (happens in the cytoplasm)
Where are ribosomes found?
Cytoplasm
What are ribosomes made of?
Mostly protein, but also some rRNA
Describe the structure of ribosomes
Two sub-units, one large sub-unit one smaller sub-unit.
How does the ribosome know where the start of the mRNA is?
By the start codon at the start of the mRNA sequence
Describe the structure of a tRNA
Describe the structure of tRNA
A clover-shaped strand of RNA held together by hydrogen bonds between complementary base pairs. It has an anticodon on one end, and the complementary amino acid on the other end.
What is the function of tRNA?
To bring the complimentary amino acid to the ribosome during translation
Make five comparisons between mRNA and tRNA
tRNA has complementary base pairing, whereas mRNA doesn’t.
tRNA has hydrogen bonding, whereas mRNA doesn’t.
tRNA has an amino acid binding site, whereas mRNA doesn’t.
mRNA has codons, and tRNA has anticodons.
mRNA can be different lengths, whereas tRNA is always the same length.
What happens before translation can begin?
mRNA binds to the ribosome
Describe how mRNA and tRNA interact during translation
tRNA with the complementary anticodon binds to the mRNA codon
How many binding sites are there in a ribosome?
2
Name the bonds that form between the amino acids in translation
Peptide
From a strand of mRNA, write the tRNA sequence: AUG-AAA-AGU-UUC-CUA
UAC-UUU-UCA-AAG-GAU
Use an amino acids reference table to transcribe the mRNA sequence into amino acids: AUG-AAA-AGU-UUC-CUA
Met-Lys-Ser-Phe-Leu
How does the ribosome know to stop translating?
It reaches a stop codon on the mRNA
What happens to the polypeptide made after translation?
The polypeptide is coiled and folded into its tertiary structure. It may also bond with other subunits to form a quaternary protein.
Explain how many polypeptides can be translated from one mRNA at once
Many ribosomes can attach to the same mRNA (at different places) at once.
Define what alleles are
Versions/forms of a gene
How do most alleles arise?
Through mutations
Define what a gene mutation is
Any change to the quantity or sequence of the DNA in a gene
When do gene mutations arise spontaneously?
During DNA replication
When can mutations be inherited?
When they occur during gamete formation (meiosis)
Describe the effect of mutagenic agents
They increase the rate of genetic mutations
Name some mutagenic agents
Tar, radiation (UV, X-Rays, Gamma Rays), Nitrogen Dioxide
Define mutation
A sudden change in the amount or the arrangement of the genetic material in the cell
State what a gene mutation is
A change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited
State what substitution mutation is
A type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base
Describe how gene mutation arise from deletion
A nucleotide is lost from the normal DNA sequence
Name the different types of mutation
Substitution, insertion, deletion
Define what a base substitution is
A nucleotide in a gene is replaced with another nucleotide with a different base
Why might a base substitution not result in a different sequence of amino acids?
Because DNA is degenerate, so the mutated codon might code for the same amino acid as the original codon
Why might one different amino acid not cause a change in the 3D shape of the protein?
Because the amino acid that has changed might not play a role in forming tertiary bonds
Define a deletion mutation
A nucleotide is lost from the normal DNA sequence
Describe how deletion mutations cause a change in the tertiary shape of the protein
A deletion causes a frame shift to the left. Therefore all codons after the deletion are read differently. This causes all amino acids after the frame shift to change.
Define an insertion mutation
A nucleotide is inserted into the normal DNA sequence.
Describe how Insertion can cause frameshift mutations
An insertion causes a frame shift to the right. Therefore all codons after the insertion are read differently. This causes all amino acids after the frame shift to change.
Describe the structure of a chromosome
A long molecule of DNA wound around proteins called histones. This complex is called chromatin.
Describe the structure of eukaryotic DNA
Linear, contains introns, is long and is associated with histones.
Describe the structure of prokaryotic DNA
Circular, does not contain introns, is short and is not associated with histones.
Mitochondrial and chloroplast DNA is similar in structure to which type of DNA?
Prokaryotic
Describe the difference in the shape of DNA in prokaryotes and eukaryotes.
Eukaryotic DNA is linear, whereas prokaryotic DNA is circular.
Describe the difference in the contents of DNA in prokaryotes and eukaryotes.
Eukaryotic DNA contains introns, whereas prokaryotic DNA does not.
Describe the difference in the length of DNA in prokaryotes and eukaryotes.
Eukaryotic DNA is longer than prokaryotic DNA.
Describe the difference in the final structure of DNA in prokaryotes and eukaryotes.
Eukaryotic DNA is associated with proteins and coiled into chromosomes, whereas prokaryotic DNA is not.
