3.4.2 DNA and protein synthesis Flashcards
mRNA: What, when, where, stability, and draw
What? Single-stranded
Bases are arranged in codons
Complementary to the DNA sequence
When? Created during transcription
Where? In the nucleus
Leaves via nuclear pore and enters the cytoplasm
Stability: Easily broken down
Only manufactured when needed
tRNA: What, where, stability, and draw
What? Single-stranded
Folded into a clover
Anticodon at one end is complementary to mRNA codon
AA binding site
Where? In the cytoplasm
Stability: More stable than mRNA; less stable than DNA
Structure of tRNA drawing (during transcription)
Draw:
1) Codon vs Anticodon
2) Why does DNA need to be chemically stable?
3) Why does mRNA need to be easily broken down?
1) An anticodon is a trinucleotide sequence complementary to that of a corresponding codon in a messenger RNA (mRNA) sequence. An anticodon is found at one end of a transfer RNA (tRNA) molecule and carries an amino acid
2) Every cell must begin with a very accurate copy of its parent’s or parents’ DNA. So it is important for the DNA to be stable, to resist change, otherwise inaccuracies will appear
3) Bc it has to be broken down as soon as it is manufactured, otherwise wasteful
Compare each feature to mRNA and tRNA
Double polynucleotide chain
Largest molecule of all 3
Double-helix molecule
Pentose sugar = deoxyribose
Organic bases: adenine, thymine, guanine, cytosine
Found mostly in nucleus
Quantity is constant for all cells in a species (except gametes)
Chemically very stable
mRNA Single stranded Similar size to DNA Single strand Ribose Uracil instead of Thymine Mostly in nucleus- can move to cytoplasm Vary Break down very quickly
tRNA Single stranded Very small Clover, folded Ribose Uracil instead of thymine Cytoplasm Vary More stable than mRNA, less stable than DNA
Protein synthesis four stages and basic steps of pp synthesis and two main stages
TRANSCRIPTION
SPLICING
TRANSLATION
PROTEIN ASSEMBLY
- DNA provides the instructions in the form of a long sequence of bases.
- A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA - a process called transcription.
- The pre-mRNA is spliced to form mRNA.
- The mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked to form a polypeptide - a process called translation.
Transcription: where one gene on the DNA is copied to the mRNA
Translation: where the mRNA joins with a ribosome and corresponding tRNA molecules bring the specific amino acids the codon codes for
Transcription: 6 marking points
1) DNA helix unwinds- to expose bases to act as a template
2) Only one chain of DNA acts as a template (compared to both in DNA repl)
3) Catalysed by DNA-Helicase
4) DNAH breaks h-bonds btw bases
5) Free mRNA nucleotides in nucleus align opposite comp exposed DNA bases
6) DNA Polymerase bonds together RNA nucleotides by creating sugar-phosphate backbone- to create new RNA polymer chain- one entire gene is copied
7) This is now Pre-mRNA- modified and then leaves nucleus through nuclear pores
Splicing in Protein Synthesis- why introns removed, protein that removes them name?
These intervening introns would prevent the synthesis of a polypeptide
Splicesome
The DNA of a gene eukaryotic cells is made up of sections called exons that code for proteins and sections called introns that do not. In the pre-mRNA of eukaryotic cells. The base sequences corresponding to the introns are removed and the functional exons are joined together during a process called splicin g. As most prokaryotic cells do not have intrans, splicing of their DNA is unnecessary.
What: Removal of introns from pre-mRNA
Where: Nucleus
What next: mRNA leaves via pores in the nuclear envelope, released into the cytoplasm and attach to ribosomes.
Transcription: How?
- Unzip DNA: DNA helicase breaks H-bonds between bases, separating the two strands
- Template copied: One strand of DNA acts as a template. RNA Polymerase creates the sugar-phosphate backbone, joining together complementary nucleotides.
- DNA rejoins: H-bonds reform
- Stop codon: RNA polymerase detaches and pre-mRNA production is complete
Translation- what, where, how
Amino acids join together via a peptide bond to form a polypeptide chain
Cytoplasm
Ribosomes
mRNA attaches
tRNA attaches
Peptide bond
Stop codon
Imp structures: Ribosomes and structural features- made of… found in… site of… prokaryotes have …
The site of protein synthesis
Found in the cytoplasm
When joined to the ER = RER
Made of rRNA + protein
Prokaryotes have 70S ribosomes Eukaryotes have 80S ribosomes
What are the two binding sites on tRNA?
Anti-codon and amino acid binding site
Translation steps: Draw diagram too
mRNA and tRNA attaches:
1) mRNA attaches to a ribosome in the cytoplasm
2) tRNA molecule with the complementary anti-codon (UAC) to the start codon moves to the ribosome and pairs with the mRNA sequence.
3) This tRNA carries a methionine
Peptide bond formation
1) Peptide bond formation requires an enzyme and ATP
2) This releases the first tRNA molecule
3) The ribosome continues to move along the mRNA strand and brings a 3rd complementary tRNA.
Peptide bond formation
1) The process continues until a complete polypeptide chain is formed.
2) The synthesis of a polypeptide continues until the ribosome reaches a STOP codon.
Polypeptide formed
1)mRNA, ribosomes and tRNA all separate
Up to 50 ribosomes can pass immediately behind the 1st, allowing for many identical polypeptides to be assembled simultaneously.
Polypeptide chain will then enter golgi apparatus for folding and modification
Nonsense vs Mis-sense vs Silent mutations
Nonsense: Base change results in the formation of one of the three stop codons
Premature stop codon = truncated protein = will affect structure and function #= won’t fold into correct shape
Mis-sense: Base change results in the formation of a different amino acid.
Polypeptide produced will differ
Protein will be different shape → won’t function properly (this is key if the protein is an enzyme)
Silent: Base change causes a change in the mRNA base sequence but due to the degenerate nature of the genetic code, the amino acid coded for is the same →no effect on the polypeptide
Insertion and Deletion mutations
Arise when a nucleotide is removed or added to the DNA sequence.
The genetic code is read in “3s”
Deletion: causes reading frame to shift to the left
Insertion: causes reading frame to shift to the right
This is called a frameshift. The sequence is completely altered and subsequent triplets are altered.
