DNA TRANSCRIPTION & TRANSLATION Flashcards
DNA
made up of two polynucleotides containing sequences of nucleotide bases that determine the sequence of amino acids
Protein is made in the ribosomes, but DNA is too large to leave the nucleus, therefore mRNA takes the code out of the nucleus through the nuclear pores.
Gene
A specific section of a chromosome that codes for an amino acid
TRICK QUESTION: why are the percentages of bases from the middle part of the chromosome and the end part different?
They are different genes! Therefore they will have different base sequences
Genetic Code
Sequence of nucleotide bases on the mRNA that code for amino acids
Universal; same codons code for same amino acid
Non-overlapping; each base is part of only one codon
Degenerate; some amino acids are coded for by several different codons
Three codons do not code for amino acid, they are called stop codons and mark the end of a polypeptide chain
Codon
Sequence of three nucleotide bases on the mRNA that code for a single amino acid
Anticodon
Group of bases complimentary to bases on the mRNA/ complimentary to codon
Introns
Non-coding, DNA, this means DNA that does not code for a protein
Degeneracy of DNA Code
some amino acids coded for by more than one triplet, the degeneracy will be on the third base, i.e. UGU, UGC, UGA, UGG all code for the same amino acid and UUU, UUC, UUA, UUG all code for the same amino acid
Differences between DNA and RNA
DNA has deoxyribose, RNA has ribose
DNA has Thymine, RNA has Uracil
DNA is double stranded, RNA is single (even tRNA is considered single stranded because it has just been folded up)
DNA is larger and longer, RNA is smaller and shorter
There is only one type of DNA, whereas there are 2 types of DNA
In DNA, The amount of Adenine = amount of Thymine and amount of Guanine = amount of Cytosine, in RNA there is variable amounts
Similarities between DNA and RNA
Both have Phosphate
Both have Adenine, Cytosine and Guanine
Both have nucleotides
Protein Synthesis has two stages
Transcription: base sequence on a particular gene is copied onto molecules of mRNA. This takes place in nucleus.
Translation: base sequence on the mRNA is used to assemble a protein using tRNA. This takes place ON ribosomes.
Transcription
Transcription Factors allow DNA Helicase to break the hydrogen bonds between the bases in the DNA molecule
The enzyme RNA Polymerase moves along one of DNA strands (this strand is called the antisense strand), known as template strand, and free nucleotides to join with the complimentary bases on the template strand
RNA POLYMERASE JOINS THE NEW NUCLEOTIDES TOGETHER TO FORM MRNA
As RNA Polymerase moves along the DNA strand, the DNA strands behind it rejoin, like a zip
When the RNA polymerase reaches a stop triplet code, it detaches, and thus we have a pre-mRNA strand
Pre-mRNA contains Introns, which do not code for an amino acid, therefore we must remove Introns using splicing
The pre-mRNA is spliced by cutting out the Introns and joining the exons together to form mRNA using proteins called snurps (REMEMBER: Prokaryotic DNA does not have Introns)
REMEMBER: DNA is made up of two polynucleotide strands, the sense strand and antisense stand, mRNA is transcribed from the DNA sense strand, which CONTAINS THE GENETIC CODE.
Also notice how in DNA Replication, DNA Polymerase was involved, here RNA Polymerase is involved, they’re not the same.
Role of RNA Polymerase in transcription
Attach nucleotides to form a strand
Why the number of bases in the DNA sense strand is the same as the number of bases in the DNA antisense strand
Complimentary sense strand
Why the number of bases in the DNA sense strand is not the same as the number of bases in the mRNA strand
DNA contains Introns which are non-coding
Role of DNA antisense strand
Provides DNA with stability
Acts as a template for a new strand in DNA Replication
Why some errors during copying of a sequence of bases are not severe
Some amino acids have more than one code, therefore error may not affect amino acid
If amino acid is changed, the new amino acid may not affect the functioning of the protein, i.e. the changed amino acid was not an important amino acid in terms of the structure and function of the protein
Translation
mRNA leaves nucleus via nuclear pore and enters ribosome where proteins are made which are to be used in the cell
Each codon on the mRNA has a complimentary anticodon on the tRNA
Each amino acid has its own tRNA molecule which it attaches to
Therefore each tRNA brings an amino acid to the mRNA which it attaches to using the anticodon via HYDROGEN BONDS
The ribosomes move along the mRNA bringing two tRNA molecules (which contain the amino acid) at a time each pairing up with corresponding codons on the mRNA
When two amino acids are together on the tRNA, they are joined together with a peptide bond using enzymes and ATP
As the two amino acids join, the ribosome moves onto the next two codons and tRNA molecules leave to collect amino acids
This continues until the ribosome reaches a stop codon
Usually there is modification after translation too, enzymes add specific methyl or phosphate groups to amino acids
Role of tRNA
Carries specific amino acid to the polypeptide chain, the amino acid is attached to the tRNA by specific aminoacyl tRNA synthase enzyme
Has an anticodon which is complimentary to the codon on the mRNA
Attaches to ribosome and holds amino acids in place
Similarities between DNA REPLICATION (making another copy of DNA) and Transcription (making protein)
Both involve Hydrogen bonds breaking between complimentary bases
Both involve complimentary nucleotides acting as templates
Differences between DNA REPLICATION (making another copy of DNA) and Transcription (making protein)
Transcription involves Uracil, Replication involves Thymine
One strand used in transcription, two strands used in replication
DNA used in replication, mRNA used in Transcription
Why only 1% of genetic information of a mammalian cell is transcribed, the rest is untouched
Only some genes are transcribed
Different proteins required by different cells, therefore only some genetic information needed
Some DNA does not code for anything (Introns)
Features of a gene that enables it to code for a protein
Gene is a length of DNA
Gene is a sequence of bases
Gene has a triplet code that codes for an amino acid
Degenerate code allows different triplets to code for the same amino acid
How different base sequences code for different proteins
Proteins are made of amino acids
Each amino acid has its own triplet code
How copying bases more than once may give rise to a difference in the protein
It Changes base sequence of later triplets
How a protein is made by a gene (SIMPLIFIED):
Growth factors allow a gene to be switched on
Unzipping of DNA involves the breaking of hydrogen bonds using DNA Helicase
Free mRNA nucleotides assemble and form complimentary base pairs
RNA Polymerase joins newly formed mRNA nucleotides
mRNA leaves nucleus via nuclear pore and enters ribosome
mRNA has a codon which is complimentary to the anticodon of a specific tRNA molecule which is associated with a specific amino acid, therefore mRN attaches to its complimentary tRNA
Once amino acids come together, peptide bonds are formed between them
tRNA detaches and collects another amino acid
Ribosome moves along mRNA
Polypeptide will be complete when a stop codon is reached
REMEMBER: We can convert one protein to another protein by re-arranging its amino acids
mRNA
Single stranded, each nucleotide contains: 1) Nitrogenous base 2) Phosphate 3) RIBOSE (not deoxyribose)
Involved in transcription of DNA bases
Leaves nucleus via the pores and enters cytoplasm where it associates with the ribosomes
mRNA is suited to its function because
Possesses correct sequence of triplets that code for specific polypeptides
Easily broken down, therefore exists only while it is needed
How tRNA is suited to it functions
End chain for attaching amino acids
Anticodon for pairing with codon of mRNA
Difference between tRNA and mRNA
mRNA has no base-pairing/mRNA is linear, whereas tRNA is folded via base-pairing
tRNA is cloverleaf shape whereas mRNA is straight
mRNA has no binding site for amino acids whereas tRNA does
more types of mRNA than tRNA
tRNA is a fixed length whereas mRNA is variable length
