1.3 RNA, transcription & translation Flashcards
How is a cell’s genotype determined?
By the sequence of DNA bases in its genes
How is a cells phenotype determined?
By the proteins that are synthesised when the genes are expressed.
What is a protein made up of?
a series of subunits called amino acids
What does the sequence of amino acids in a protein determine?
the structure and function of the protein
What is the order of amino acids in a protein determined by?
the sequence of bases in a DNA molecule
How many bases code for one amino acid?
3
What is a gene?
a small section of DNA that codes for a particular protein
How many genes in a cell are expressed?
only a fraction
What is meant by expressed?
Used to make a protein
What happens to the other genes that are not expressed?
they are switched off during differentiation
Which genes are expressed are influenced by what?
intra- and extra-cellular environmental factors
Give examples of intra- and extra-cellular environmental factors?(3)
diet
activity levels
stress
What can intra- and extra-cellular environmental factors trigger?
changes in pH, chemical and hormone production which in turn switches genes on or off.
What is gene expression controlled by?
the regulation of transcription and translation
What is transcription?
It is the synthesis of mRNA from a section DNA.
What is translation?
It is the synthesis of a polypeptide (protein) chain using the sequence of bases on the mRNA.
What does both transcription and translation require?
a second type of nucleic acid called RNA.
What is a triplet in mRNA called?
A codon
What is RNA made up of?
nucleotides
What is a RNA nucleotide made up of?
a phosphate group
an organic base
a ribose sugar
How many types of RNA are there and how are they different?
3
Due to how the nucleotides can be linked together
What is mRNA formed from and what does it do?
the appropriate section of DNA strand and carries a complementary copy of the DNA code from the nucleus to a ribosome
What are the 3 different types of RNA?
mRNA (messenger RNA)
rRNA (ribosomal RNA)
tRNA (transfer RNA)
What does rRNA do?
combines with proteins to form the ribosome
What does each tRNA do?
carries a specific amino acid to a ribosome to build protein
what must you remember about tRNA?
Anticodon
What base does mRNA have instead of thymine?
uracil
What are the 4 mRNA bases?
adenine
uracil
guanine
cytsine
How many strands does mRNA have?
1 (single strand)
What is the first stage of protein synthesis?
transcription
What happens during transcription?
a complementary copy is made of the relevant section of DNA(gene). This copy is known as mRNA
What is the 2 important regions on the gene and where are they located?
promoter (start codon) at start of gene and terminator (stop codon)
Describe stage 1 of transcription?
Transcription begins at a region known as the promoter.
Describe stage 2 of transcription?
An enzyme called RNA polymerase attaches to the promoter region.
Describe stage 3 of transcription?
The DNA unwinds and the weak hydrogen bonds break, causing the 2 DNA strands to separate.
Describe stage 4 of transcription?
Free RNA nucleotides line up against their complementary base on the coding DNA strand (following the bade paring rule). RNA polymerase adds the nucleotides to the 3’ end of the forming mRNA strand.
Describe stage 5 of transcription?
Weak hydrogen bonds form between the base pairs and strong sugar-phosphate bonds form between the adjacent RNA nucleotides, forming a strand of mRNA called the primary transcript. This process stops when the terminator region is reached.
Before leaving the nucleus, what happens to the primary transcript of mRNA?
Has to be modified to form the mature transcript
How is the primary transcript modified?
Involves removing any of the non-coding sections called introns, leaving behind the coding regions called exons.
What is RNA splicing?
the process that joins together (splices) the remaining exons to form a continuous sequence called the mature transcript.
What happens after the mature transcript has formed?
it leaves the nucleus to travel to the ribosome, moving onto the next stage of protein synthesis called translation
What is the second stage of protein synthesis?
tranlation
What is mRNA used to synthesise?
a polypeptide chain in the ribosome
What does each codon code for?
one amino acid
Where is tRNA located?
in the cytoplasm
How many strands is tRNA?
1 (single stranded)
What is different about tRNA structure?
it is folded into a 3D structure, held together by hydrogen bonds due to base pairing
Why is tRNA held together by hydrogen bonds?
Due to base pairing
What is at one end of each tRNA molecule?
a triplet of bases called an anticodon
What is at the opposite end of each tRNA molecule?
an amino acid which the tRNA molecule is responsible for carrying to a ribosome
What does the anticodon determine?
the specific amino acid that each tRNA molecule will pick up and then carry to a ribosome
Give 3 similarities between mRNA and tRNA?
Both are composed of nucleotides
Both are single stranded
Both have bases A,U,C,G
Give 3 differences between mRNA and tRNA?
tRNA is folded into a 3D structure in such a way that hydrogen bonds form between many of its nucleotides./ mRNA is linear.
tRNA has only one exposed triplet of bases/ mRNA has all triplets exposed.
tRNA carries amino acids/mRNA does not
Describe stage 1 of translation?
The ribosome binds to the 5’ end of the mRNA at the start codon (AUG)
Describe stage 2 of translation?
Next a tRNA carrying the amino acid (methionine) lines up against its complementary codon on the mRNA. Hydrogen bonds form between the anticodon (UAC) and the start codon.
Describe stage 3 of translation?
Another tRNA molecule brings a specific amino acid to the ribosome and lines up its complementary anticodon against the next mRNA codon. Hydrogen bonds form between bases.
Describe stage 4 of translation?
A peptide bond then forms between the first two amino acids.
Describe stage 5 of translation?
The ribosome then moves along one codon exposing the next codon to a tRNA anticodon
Describe stage 6 of translation?
This process is repeated many times until a stop codon is reached and a complete polypeptide chain has been translated.
What is a start codon?
a triplet of bases (usually AUG) on mRNA that initiates the translation process.
What does a stop codon do?
A stop codon does not code for an amino acid but signifies that the polypeptide chain is complete. (usually UAA,UAG AND UGA.
What are different mRNA molecules produced from?
The same primary transcript
What can the same gene be used to make?
Several different proteins
By what two processes can the same gene make several different proteins?
Alternative RNA Splicing
Post-translational modification
When does alternative RNA splicing occur?
At the end of transcription when the introns are removed to make the mature transcript
Describe alternative RNA splicing?
Different RNA Segmants can be treated as introns, and the resulting exons can be spliced together in a different order to form different proteins from the same original gene.
How can a polypeptide chain be modified after it has been formed in translation?(2)
Cleavage
Molecular addition
Describe what cleavage is ?
The single polypeptide chain can be cut (cleaved) and/or combined with other polypeptide chains to form different protein structures
Describe what molecular addition is?
A polypeptide chain can have other molecule added such as carbohydrates and phosphate groups
What is a glycoprotein?
Addition of a protein and a carbohydrate
Give an example of a glycoprotein?
Mucus
