DNA and Protein synthesis Flashcards
State what are the nucleic acids and what makes up a nucleic acid
DNA and RNA are nucleic acids.
Monomers of nucleic acids: Nucleotides
Polymers: Polynucleotides: DNA and RNA
Describe the structure of a nucleotide
- Nitrogenous base
- Pentose sugar: Ribose/ Deoxyribose
- Phosophate group: Negatively charged, Making DNA a negatively charged molecule
What is a nuceloside
Nitrogenous base + Pentose sugar
Describe the complementary base pairing
- Purines: 2 rings
Adenine and Guanine - Pyramidine: 1 ring
Cytosine and thyamine - Purines always pairs up with pyramidine: DNA will have the same width throught 2ring+1ring= 3 ring
- Bases are paired with hydrogen bonds:
A = T : 2 hydro gen bonds
C ≡ G : 3 hydrogen bonds
Purine: pure as gold
Describe the structure of RNA
RNA nucelotides include:
1. Nitrogenous bases: A, U, C, G
2. Ribose sugar
3. Pentose sugar
It is a single stranded polynucleotide that forms a single helix
Describe the structure of ATP
ATP= adenosine triphosphate
adenine+base= adenosine
Consists of:
1. 1 nitrogenous base
2. 1 ribose sugar
3. 3 Phosphate groups
Describe DNA nucleotide
Consists of:
1. 1 nitrogenous base: A, T, C, G
2. Deoxyribose sugar
3. A phosphate group
Desribe the structure of DNA
- Complementary base pairing between opposite strands
- Sugar phosphate backbone
Alternating sugar and phosphate group linked by phosphodiester bonds - Two stands of polynucleotides are antiparallel: run in opposite direction
One strand runs from 5’ to 3’, the other from 3’ to 5’ - Hydrogen bonding present between complementary bases
Enzymes needed for semiconservative replication
Helicase:
* Breaks hydrogen bonds to seperate 2 DNA strands
DNA polymerase:
* To synthesise a new strand of DNA IN 5’ to 3’ direction
* To catalyse formation of phosphodiester bonds
DNA ligase:
* To join the okazaki fragments together
Explain the steps of DNA replication
- Unwinding: The DNA double helix unwinds entirely.
- Helicase Action: Helicase breaks hydrogen bonds, separating the two strands, which act as templates.
- Activated Nucleotides: Free nucleotides, activated with two extra phosphates, form complementary base pairs with the template strands.
-
DNA Polymerase: DNA polymerase attaches to both parental strands, synthesizing new DNA in the 5’ to 3’ direction.
It removes two phosphates from nucleotides, forms phosphodiester bonds, and proofreads the new DNA. -
Leading and Lagging Strands:
- The leading strand is synthesized continuously.
- The lagging strand is synthesized in Okazaki fragments, later joined by DNA ligase. ( DNA molecules always formed from 5’ to 3’ direction)
- Result: Two semi-conservative DNA molecules, each with one original and one new strand.
What is transcription and where does it occur?
DNA is copied to mRNA
Takes place in the nucleus
What is splicing and where does it occur?
Splicing is the removal of introns (non-coding genes) from the mRNA
Takes place in the nucleus
What is transalation and where does it occur?
mRNA is transalated into a polypeptide chain
In the ribosomes at the RER/Cytoplasm
How many probabilities are there of each position of triplet codes?
4^3=64 triplet codes can only code for 20 aminoacids
State properties of codons
- They are degenerate ( Multiple triplet code codes for the same amino acid)
- They are universal: The same codons code for the same amino acids across almost all living organisms.
- There are stop and start codons
Describe the structure of a ribosome
rRNA + proteins = ribosome
Site of translation (protein synthesis)
Two subunits:
Small: mRNA binding site
Large: Contains tRNA binding sites
3 sites:
A (Aminoacyl site) – tRNA entry
P (Peptidyl site) – Peptide bond formation
E (Exit site) – tRNA exits (Therefore only two sites are occuiped at the same time by the tRNA)
Explain transcription
- DNA unwinds (only the gene region).
- Helicase breaks H-bonds, separating strands.
- RNA nucleotides pair with the template strand via complementary base pairing.
- RNA polymerase catalyzes mRNA synthesis (5’ → 3’ direction).
- Pre-mRNA (with introns & exons) is formed.
- RNA splicing removes introns; exons form mature mRNA, which exits the nucleus.
Explain transalation
- mRNA binds to ribosome at the start codon (AUG).
- tRNA with amino acids binds to ribosome via anticodon-mRNA complementary pairing.
- Peptide bond forms between amino acids (catalyzed by peptidyl transferase).
- Ribosome moves 5’ → 3’, adding amino acids one by one.
- tRNA detaches & is recycled.
- Polypeptide is released when a stop codon (UAA, UAG, UGA) is reached.
What is gene mutation
Gene Mutation – Changes in the DNA base sequence, leading to altered codons and possibly new alleles, this results in changes in structures of the protein formed.
Describe the gene mutations
Substitution: One base replaced by another
Insertion/Deletion – Extra/missing bases cause:
Frameshift Mutation: Shifts the reading frame, altering all subsequent codons → non-functional protein.
If 3 nucleotides are deleted/added, one amino acid is lost/added, but the reading frame remains intact.
Ratio of the bases
its 1:1 among the pairs
A:T
C:G
Meselson & Stahl’s Experiment (Semi-Conservative Replication)
- Grew E. coli in heavy nitrogen (15N) → DNA became heavy
- Transferred bacteria to light nitrogen (14N) → New DNA incorporated 14N
Before Replication (All in 15N Medium)
DNA Molecules: 100% heavy
DNA Strands: Both strands in each molecule are heavy (15N-15N)
After 1st Replication (In 14N Medium)
DNA Molecules: 100% have one heavy (15N) strand and one light (14N) strand
No fully heavy or fully light DNA yet
After 2nd Replication (In 14N Medium)
DNA Molecules:
50% have one heavy (15N) strand and one light (14N) strand
50% have both strands light (14N-14N)
Conclusion:
Each new DNA molecule keeps one strand from the original DNA and one newly made strand, proving semi-conservative replication.
