2.1.3 Nucleotides and nucleic acids Flashcards
What are the three components of a nucleotide?
- A pentose sugar - contains 5 carbon atoms
- A nitrogenous base
- A phosphate group
How do nucleotides join together to form a polynucleotide?
- Via condensation reactions, forming phosphodiester bonds between the phosphate of one nucleotide and the sugar of another.
- This forms the sugar-phosphate backbone.
What are the components of a DNA nucleotide?
- Deoxyribose sugar
- Phosphate group
- One of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
What are key features of the DNA that support its function?
- Sugar-phosphate backbone - protects coding bases on inside of helix
- Double stranded - allows strands to act as templates in repilcation/protein synthesis
- Large molecule - stores a lot of information
- Double helix - compact
- Complementary base pairing - allows accurate replication
- Weak H bonds - allows strands to seperate in replication
- Strands run anti-parallel.
What is the difference between purines and pyrimidines?
Purines = Larger bases that contain two carbon ring structures.
Pyrimidines = Smaller bases that contain one carbon ring structures.
Name the two purines
Adenine & Guanine
Name the three pyrimidines
Cytosine, Thymine & Uracil
What are the base pairing rules in DNA?
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds
- Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds
How is RNA structurally different from DNA?
- Ribose sugar instead of deoxyribose
- Single-stranded
- Shorter than DNA
- Contains Uracil (U) instead of Thymine (T) base
What is semi-conservative replication?
Each new DNA molecule contains one original strand and one newly synthesised strand.
What are the main steps in DNA replication?
- DNA helicase breaks H bonds between complementary bases. This unwinds the double helix, seperating strands
- Free nucleotides pair with complementary bases on each template strand.
- DNA polymerase joins nucleotides via condensation reactions in the 5’ to 3’ direction, forming phosphodiester bonds.
- The lagging strand (3’ to 5’ strand) is synthesised in short fragments using DNA ligase.
- Two identical DNA molecules are formed (semi-conservative replication).
What was the aim of the Meselson-Stahl experiment?
To determine whether DNA replication was conservative or semi-conservative.
What did Meselson and Stahl observe after one replication cycle?
All DNA molecules had intermediate density → one heavy and one light strand.
Supports semi-conservative replication
What did the second replication cycle show in the Meselson-Stahl experiment?
50% DNA with one heavy and one light strand (intermediate band)
50% DNA with two light strands (lighter band)
Confirms semi-conservative replication.
Why does DNA need to be tightly packed in eukaryotic cells?
Must be tightly packed to fit inside the nucleus.
This allows for efficient storage and organisation.
What role do histones play in DNA condensation?
Histones are proteins that DNA wraps around to form DNA-histone complexes.
These structures coil further to form chromatin, which condenses into chromosomes.
What is a chromosome made of?
Each chromosome is made from a single, long DNA molecule.
What is a codon?
A codon is a sequence of three mRNA bases that codes for a specific amino acid.
How does a DNA triplet relate to a codon?
A DNA triplet is transcribed into an mRNA codon.
What does the term ‘genetic code’ refer to?
The genetic code is the sequence of DNA bases that determines the sequence of amino acids in a protein.
What are the three key features of the genetic code?
- Universal - Each DNA triplet codes for the smae amino acid in all organisms
- Non-overlapping - Each base in the DNA sequence is only read once
- Degenerate - Most amino acids are coded for by more than one codon.
What is a gene?
A gene is a short section of DNA that codes for a polypeptide, located at a specific locus on a chromosome.
What is the role of messenger RNA (mRNA) in protein synthesis?
mRNA carries genetic information from DNA to ribosomes.
It is made during transcription.
What are the key structural features of mRNA?
- Single-stranded, linear molecule
- Contains codons (triplets of bases)
- Complementary to a DNA template strand
- Small enough to exit the nucleus through nuclear pores
What is the role of transfer RNA (tRNA) in protein synthesis?
tRNA transports amino acids to ribosomes to build up a polypeptide chain
It matches amino acids to the codons using its anticodons during translation.
What are the structural features of tRNA?
- Single-stranded molecules but folded into a colver-leaf shape
- Hydrogen bonds between complementary bases help hold its shape (add stability)
- Has an anticodon at one end that pairs with mRNA codons.
- Contains an amino acid binding site at the opposite end.
Compare mRNA and tRNA in terms of structure and function.
- mRNA carries genetic info from DNA to ribosome but tRNA transports aa to the ribosomes to build proteins
- mRNA has a linear shape, tRNA has a clover-shape
- mRNA is longer than tRNA
- tRNA has H bonds but mRNA has nor H bonds
- tRNA contains a binding site for amino acid but mRNA does not.
- mRNA has a codon whereas tRNA has an anticodon
In which part of the cell does transcription take place?
In the nucleus (for eukaryotic cells).
What are the main steps of transcription?
- Transcription begins with RNA polymerase binding to DNA
- The H bonds between DNA bases break, seperating the two strands
- One of the strand is then used as a template (anti-sense strand)
- Free RNA nucleotides pair with DNA via complementary base pairing.
- Uracil pairs with adenine, cytosine pairs with guanine
- RNA polymerase joins nucleotides using phosphodiester bonds.
- A complementary mRNA strand is formed (carrying same base sequence as sense strand)
- RNA polymerase reaches a stop codon and detaches.
- DNA rewinds, and mRNA exits the nucleus.
Where does translation take place in the cell?
In the ribosome.
What are the main steps of translation?
- The ribosome attaches to the mRNA strand at the start codon.
- A tRNA with complementary anticodon binds to the codon, bringing a specific amino acid.
- A second tRNA binds to the next codon with its specific amino acid.
- A peptide bond forms between the amino acids (uses ATP).
- First tRNA detaches and is recycled.
- Ribosome moves along mRNA, repeating the process.
- Continues until a** stop codon** is reached.
- Polypeptide is released to fold into a functional protein.
At any point during translation, how many tRNA molecules can be attached to the ribosome?
Two tRNA molecules
What is ATP and why is it important for cells?
