Genetics: Transcription And Translation Flashcards
(18 cards)
How is DNA stored in Eukaryotic cells?
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.
The DNA molecule is really long, so it has to be wound up so it can fit into the nucleus. It’s wound around proteins called histones. Histone proteins also help to support the DNA. The DNA (and protein) is then coiled up very tightly to make a compact chromosome.
(The mitochondria and chloroplasts in eukaryotic cells also have their own DNA. This is pretty similar to prokaryotic DNA because it’s circular and shorter than DNA molecules in the nucleus. It’s not associated with histone proteins.)
How is DNA stored in Prokaryotic cells?
Prokaryotes also 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.
What is the genetic code?
The genetic code is the sequence of base triplets (codons) in mRNA which code for specific amino acids. In the genetic code, each base triplet is read in sequence, separate from the triplet before it and after it. Base triplets don’t share their bases — the code is non-overlapping.
The genetic code is also universal - the same specific base triplets code for the same amino acids in all living things.
What is meant when the genetic code is described as ‘degenerate’?
There are more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets).
This means that some amino acids are coded for by more than one base triplet, e.g. tyrosine can be coded for by UAU or UAC. Not all triplets code for amino acids though. For example, some triplets are used to tell the cell when to stop production of a protein — these are called stop signals. They’re found at the end of the mRNA. E.g. UAG is a stop signal.
What is transcription?
Where the DNA code is copied into a molecule called mRNA
What is translation?
Where the mRNA joins with an organelle called a ribosome and the code it carries is used to synthesise a protein
What is the function of mRNA (messenger)?
mRNA is made during transcription. It carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation. mRNA is a single polynucleotide strand. In mRNA, groups of three adjacent bases are usually called codons
What is the function of tRNA (transfer)?
tRNA is involved in translation. It carries the amino acids that are used to make proteins to the ribosomes. tRNA is a single polynucleotide strand that’s folded into a clover shape.
Hydrogen bonds between specific base pairs hold the molecule in this shape. Every tRNA molecule has a specific sequence of three bases at one end called an anticodon. It also has an amino acid binding site at the other end.
Where does transcription take place in prokaryotes and eukaryotes?
During transcription an mRNA copy of a gene is made from DNA. In eukaryotic cells, transcription takes place in the nucleus. Prokaryotes don’t have a nucleus, so transcription takes place in the cytoplasm.
What are the four stages of transcription?
- RNA polymerase attaches to the DNA
- Complementary mRNA is formed
- RNA polymerase moves down the DNA strand
- RNA Polymerase reaches the stop signal
Stage one of transcription:
Transcription 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
Stage two of transcription:
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 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
Stage three of transcription:
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
Stage four of transcription:
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
What is an intron?
A segment of a DNA or RNA molecule which does not code for proteins and interrupts the sequence of genes.
What is an exon?
A segment of a DNA or RNA molecule containing information coding for a protein or peptide sequence.
How is mRNA ‘edited’?
In eukaryotes, the introns and exons are both copied into mRNA during transcription. mRNA strands containing introns and exons are called pre-mRNA. A process called splicing then occurs - introns are removed and the exons joined together — forming mRNA strands. This takes place in the nucleus. The mRNA then leaves the nucleus for the next stage of protein synthesis (translation).
In prokaryotes, mRNA is produced directly from the DNA - without splicing taking place. There’s no need for splicing because there are no introns in prokaryotic DNA.
What is the process involved 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.
A tRNA molecule (carrying an amino acid), with an anticodon that’s 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.
The two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away, leaving its amino acid behind.
A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away. This process continues, producing a chain of linked amino acids (a polypeptide chain), until there’s a stop signal on the mRNA molecule.
