Topic 4 — A: DNA, RNA and Protein Synthesis Flashcards
How is DNA stored?
- structure of DNA is the same in all organisms, but organisms, eukaryotic and prokaryotic cells store DNA in slightly different ways.
DNA of Eukaryotic cells:
contain linear DNA molecules that exist as chromosomes — thread-like structures, each made up of one long molecule of DNA and
its associated proteins.
- Chromosomes are found in the nucleus.
shape of DNA molecules in eukaryotic cells:
long, so it has to be wound up so it can
fit into the nucleus. It’s wound around proteins called histones.
what do Histone proteins do?
help to support the DNA.
The DNA (and protein) is then coiled up very tightly to make a compact chromosome.
DNA in mitochondria and chloroplasts:
have their own DNA.
- similar to prokaryotic DNA but circular and shorter than DNA molecules in the nucleus + not associated with histone proteins.
Prokaryotic cells DNA:
- carry DNA as chromosomes
- but the DNA molecules are shorter and circular. The DNA isn’t wound around histones — it condenses to fit in the cell by supercoiling.
process of DNA coiling:
DNA double helix → circular chromosome → coiled chromosome → super coiled to fit in cell
how much of DNA carries genetic info?
Only a small amount of the DNA in a cell carries genetic information.
The most important parts of a DNA molecule are the genes.
what does DNA contain?
genes
what is a gene?
sequence of DNA bases that codes for either a polypeptide or functional RNA
- The sequence of amino acids in a polypeptide forms the primary structure of a protein
what do Different polypeptides have?
a different number and order of amino acids.
what does the order of bases in a gene determine?
the order of amino acids in a particular polypeptide
what is Each amino acid is coded by?
a sequence of three bases in a gene called a triplet or codon
making a polypeptide (first stage of protein synthesis)
DNA is first copied into messenger RNA (mRNA).
what do Genes that don’t code for a polypeptide code for?
functional RNA
what is Functional RNA?
RNA molecules other than mRNA, which perform special tasks during protein synthesis, e.g. tRNA and ribosomal RNA (rRNA), which forms part of ribosomes.
genome definition:
The complete set of genes in a cell
what is a proteome?
full range of proteins that the cell is able to produce
what does nuclear DNA in eukaryotes NOT code for?
doesn’t
p code for polypeptides
what do genes that do code for polypeptides also contain?
sections that don’t code for amino acids (introns)
introns within a gene:
There can be several introns within a gene and their purpose isn’t known for sure.
how are Introns in eukaryotes removed?
protein synthesis — so they don’t affect the
amino acid order.
does Prokaryotic DNA have introns?
no
exons meaning:
All the bits of a gene that do code for amino acids
what do eukaryotic DNA contain outside of genes?
contains regions of multiple repeats outside of
genes (DNA sequences that repeat)
- These areas don’t code for amino acids either, so they’re called non-coding multiple repeats.
Alleles meaning:
A gene can exist in more than one form. These forms are called alleles.
why do alleles code for slightly different versions of the same polypeptide?
order of bases in each allele is slightly different
example of genes coding:
The gene that codes for blood type exists as one of three alleles —
one codes for type O, another for type A and the other for type B.
where is DNA stored in eukaryotic cell nucleus?
stored as chromosomes
how many pairs of chromosomes?
23 pairs of chromosomes, 46 in total
homologous pairs meaning?
Pairs of matching chromosomes (e.g. the 1s)
size of chromosomes in a homologous pair!
both chromosomes are the same size and have the same genes, although they could have different alleles.
where are alleles coding for the same characteristic found?
found at the same fixed position (locus) on each chromosome in a homologous pair.
What is protein synthesis?
production of proteins (polypeptides) from the
information contained within a cell’s DNA.
- also known as polypeptide synthesis.
what does polypeptide synthesis involve?
- two main stages
1. transcription where the DNA code is copied into a molecule called mRNA
2. translation - where the mRNA joins with a ribosome and the code it carries is used to synthesise a protein
what is RNA?
single polynucleotide strand
- contains uracil (U) as a base instead of thymine
-
what does Uracil always pair with during protein synthesis?
adenine
types of RNA!
mRNA and tRNA.
what does (mRNA) stand for?
Messenger RNA
when is mRNA made?
during transcription
what does mRNA do?
carries genetic code from DNA to the ribosomes, where it’s used to make a protein during translation.
structure of mRNA:
single polynucleotide strand.
what are the groups of three adjacent bases usually in mRNA?
codons (they’re sometimes called triplets or base triplets).
when is tRNA used?
involved in translation.
what does (tRNA) stand for!
Transfer RNA
what does tRNA do?
carries amino acids that are used to make proteins to the ribosomes.
structure of tRNA:
single polynucleotide strand that’s folded into a clover shape.
what do Hydrogen bonds do in tRNA?
Hydrogen bonds between specific base pairs hold the molecule in this shape.
number of bases in tRNA molecule:
tRNA molecule has a specific sequence of three bases at one end (anticodon). It also has an
amino acid binding site at the other end.
how are proteins synthesised?
using the instructions in DNA.
Protein synthesis involves transcription and translation.
How are proteins synthesised?
using the instructions in DNA
What does Protein synthesis involve?
transcription and translation.
What is Transcription?
During transcription an mRNA copy of a gene is made from DNA.
Where does transcription take place in eukaryotic cells?
In the nucleus
Where does transcription take place in prokaryotes?
Cytoplasm
Step 1 of transcription:
RNA polymerase attaches to the DNA
How does RNA polymerase attach to the DNA?
- starts when RNA polymerase
(an enzyme) attaches to the DNA
double‑helix at the beginning of a gene.
In eukaryotes, the hydrogen bonds
between the two DNA strands in the gene
are broken by a DNA helicase attached to
the RNA polymerase. This separates the
strands, and the DNA molecule uncoils
at that point, exposing some of the bases.
One of the strands is then used as a
template to make an mRNA copy
Step 2 of transcription:
Complementary mRNA is formed
How is Complementary mRNA formed?
The RNA polymerase lines up free RNA
nucleotides alongside the exposed bases
on the template strand. The free bases are
attracted to the exposed bases. Specific,
complementary base pairing (see p. 54)
means that the mRNA strand ends up
being a complementary copy of the DNA
template strand (except the base T is
replaced by U in RNA). Once the RNA
nucleotides have paired up with their
specific bases on the DNA strand, they’re
joined together by RNA polymerase,
forming an mRNA strand
Step 3 of transcription:
RNA polymerase moves down the DNA strand
How does RNA polymerase move down the DNA strand?
The RNA polymerase moves along the DNA, assembling the mRNA strand. The hydrogen
bonds between the uncoiled strands of DNA re‑form once the RNA polymerase has passed
by and the strands coil back into a double‑helix
Step 4 of transcription:
RNA polymerase reaches stop signal
How does RNA polymerase reaches its stop signal?
When RNA polymerase
reaches a particular sequence
of DNA called a stop signal,
it stops making mRNA and
detaches from the DNA.
In eukaryotes, mRNA moves
out of the nucleus through a
nuclear pore and attaches to
a ribosome in the cytoplasm,
where the next stage of
protein synthesis takes place
Where does transcription produce products?
In eukaryotes and prokaryotes
Editing mRNA in eukaryotes:
introns and exons are both copied into mRNA during transcription.
what is pre-mRNA?
mRNA strands containing introns and exons
What happens after the introns and exons are copied into mRNA in eukaryotes?
Splicing occurs
- introns are removed and
the exons joined together — forming mRNA strands
Where does editing mRNA happen in eukaryotes?
in the nucleus
What happens after exons are joined together?
mRNA then leaves the nucleus for the next stage of protein synthesis (translation).
How is mRNA produced in prokaryotes?
produced directly from the DNA — without splicing taking place.
- no need for splicing as no introns in prokaryotic DNA.
What is the second stage of protein synthesis?
Translation
Where does translation occur in eukaryotes and
prokaryotes?
occurs at the ribosomes in the cytoplasm.
What happens to amino acids during translation?
amino acids are joined together to make a polypeptide chain
(protein), following the sequence of codons carried by the mRNA.
How do amino acids join together in translation?
The mRNA attaches itself to a ribosome and transfer RNA (tRNA) molecules carry amino acids to it. ATP provides the energy needed for the
bond between the amino acid and the tRNA molecule to form.
What does the tRNA (carrying an amino acid), with an anticodon do?
- is complementary to the first codon on the mRNA
- attaches itself to the mRNA by complementary base pairing.
- A second tRNA molecule attaches itself to
the next codon on the mRNA in the same way.
What are two amino acids attached to the tRNA molecules joined by?
joined by a peptide bond.
- The first tRNA molecule moves away, leaving its amino acid behind.
What happens to the third tRNA molecule in translation?
- binds to the next codon on the mRNA
Its amino acid binds to the first two and the second tRNA molecule moves
away.
- process continues, producing a chain of linked amino acids
(a polypeptide chain), until there’s a stop signal (see next page) on the mRNA molecule.
Last stage in translation:
The polypeptide chain (protein) then moves away from the ribosome and translation is complete.
What is the genetic code?
sequence of base triplets (codons) in mRNA which code for specific amino acids.
How is each base triplet read in the genetic code?
read in sequence, separate from the triplet before it and after it.
What is a non overlapping code?
Base triplets don’t share their bases
How is the genetic code degenerate?
- more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets).
- e.g some amino acids are coded for by more than one base triplet
Example of some amino acids being coded for by more than one base triplet:
tyrosine can be coded for by UAU or UAC
Do all triplets code for amino acids?
No. some triplets are used to tell the cell
when to stop production of a protein — these are called stop signals found at end of mRNA
Example of stop signal:
UAG
Where are start signals?
the start of the mRNA which tell the cell when to start protein production, but these code for a specific amino acid called methionine
What is universal?
- genetic code is universal
- the same specific base triplets code for the same amino acids in all living things. E.g. UAU codes for tyrosine in all organisms.
To investigate how two new drugs affect nucleic acids and their role in
protein synthesis, bacteria were grown in normal conditions for a few
generations, then moved to media containing the drugs. After a short period
of time, the concentration of protein and complete strands of mRNA in the
bacteria were analysed. The results are shown in Figure 2:
Figure 2 on the previous page shows that both mRNA and protein
concentration were lower in the presence of drug 1 compared to the
no-drug control. This suggests that drug 1 affects the production of full
length mRNA, so there’s no mRNA for protein synthesis during translation.
mRNA production in the presence of drug 2 was unaffected,
but less protein was produced — 3 mg cm–3 compared to 8 mg cm–3
.
This suggests that drug 2 interferes with translation. mRNA was produced,
but less protein was translated from it.
Further tests to establish the nature of the two drugs were carried
out. Drug 1 was found to be a ribonuclease (an enzyme that digests RNA).
This could explain the results of the first experiment — most strands of
mRNA produced by the cell would be digested by drug 1, so couldn’t be
used in translation to make proteins.
Drug 2 was found to be a single-stranded, clover-shaped molecule
capable of binding to the ribosome. Again, this helps to explain the results
from the first experiment — drug 2 could work by binding to the ribosome,
blocking tRNAs from binding to it and so preventing translation
