2.1.3 Nucleotides and nucleic acids

Question 1

What are the three components of a nucleotide?

Answer 1

1. A pentose sugar - contains 5 carbon atoms
2. A nitrogenous base
3. A phosphate group

Question 2

How do nucleotides join together to form a polynucleotide?

Answer 2

• Via condensation reactions, forming phosphodiester bonds between the phosphate of one nucleotide and the sugar of another.
• This forms the sugar-phosphate backbone.

Question 3

What are the components of a DNA nucleotide?

Answer 3

• Deoxyribose sugar
• Phosphate group
• One of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

Question 4

What are key features of the DNA that support its function?

Answer 4

• 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.

Question 5

What is the difference between purines and pyrimidines?

Answer 5

Purines = Larger bases that contain two carbon ring structures.
Pyrimidines = Smaller bases that contain one carbon ring structures.

Question 6

Name the two purines

Answer 6

Adenine & Guanine

Question 7

Name the three pyrimidines

Answer 7

Cytosine, Thymine & Uracil

Question 8

What are the base pairing rules in DNA?

Answer 8

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds
• Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds

Question 9

How is RNA structurally different from DNA?

Answer 9

• Ribose sugar instead of deoxyribose
• Single-stranded
• Shorter than DNA
• Contains Uracil (U) instead of Thymine (T) base

Question 10

What is semi-conservative replication?

Answer 10

Each new DNA molecule contains one original strand and one newly synthesised strand.

Question 11

What are the main steps in DNA replication?

Answer 11

1. DNA helicase breaks H bonds between complementary bases. This unwinds the double helix, seperating strands
2. Free nucleotides pair with complementary bases on each template strand.
3. DNA polymerase joins nucleotides via condensation reactions in the 5’ to 3’ direction, forming phosphodiester bonds.
4. The lagging strand (3’ to 5’ strand) is synthesised in short fragments using DNA ligase.
5. Two identical DNA molecules are formed (semi-conservative replication).

Question 12

What was the aim of the Meselson-Stahl experiment?

Answer 12

To determine whether DNA replication was conservative or semi-conservative.

Question 13

What did Meselson and Stahl observe after one replication cycle?

Answer 13

All DNA molecules had intermediate density → one heavy and one light strand.

Supports semi-conservative replication

Question 14

What did the second replication cycle show in the Meselson-Stahl experiment?

Answer 14

50% DNA with one heavy and one light strand (intermediate band)

50% DNA with two light strands (lighter band)

Confirms semi-conservative replication.

Question 15

Why does DNA need to be tightly packed in eukaryotic cells?

Answer 15

Must be tightly packed to fit inside the nucleus.
This allows for efficient storage and organisation.

Question 16

What role do histones play in DNA condensation?

Answer 16

Histones are proteins that DNA wraps around to form DNA-histone complexes.
These structures coil further to form chromatin, which condenses into chromosomes.

Question 17

What is a chromosome made of?

Answer 17

Each chromosome is made from a single, long DNA molecule.

Question 18

What is a codon?

Answer 18

A codon is a sequence of three mRNA bases that codes for a specific amino acid.

Question 19

How does a DNA triplet relate to a codon?

Answer 19

A DNA triplet is transcribed into an mRNA codon.

Question 20

What does the term ‘genetic code’ refer to?

Answer 20

The genetic code is the sequence of DNA bases that determines the sequence of amino acids in a protein.

Question 21

What are the three key features of the genetic code?

Answer 21

• 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.

Question 22

What is a gene?

Answer 22

A gene is a short section of DNA that codes for a polypeptide, located at a specific locus on a chromosome.

Question 23

What is the role of messenger RNA (mRNA) in protein synthesis?

Answer 23

mRNA carries genetic information from DNA to ribosomes.
It is made during transcription.

Question 24

What are the key structural features of mRNA?

Answer 24

• Single-stranded, linear molecule
• Contains codons
• Complementary to a DNA template strand
• Small enough to exit the nucleus through nuclear pores

Question 25

What is the role of **transfer RNA (tRNA)** in protein synthesis?

Answer 25

tRNA transports **amino acids to ribosomes** to build up a polypeptide chain It matches amino acids to the codons using its **anticodons** during translation.

Question 26

What are the structural features of tRNA?

Answer 26

* **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.

Question 27

Compare mRNA and tRNA in terms of structure and function.

Answer 27

* 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**

Question 28

In which part of the cell does transcription take place?

Answer 28

In the nucleus (for eukaryotic cells).

Question 29

What are the main steps of transcription?

Answer 29

1. Transcription begins with **RNA polymerase** binding to DNA 2. The H bonds between DNA bases break, seperating the two strands 3. One of the strand is then used as a **template** (anti-sense strand) 4. Free RNA nucleotides pair with DNA via complementary base pairing. 5. Uracil pairs with adenine, cytosine pairs with guanine 6. **RNA polymerase** joins nucleotides using phosphodiester bonds. 7. A **complementary mRNA strand** is formed (carrying same base sequence as sense strand) 8. RNA polymerase reaches a **stop codon** and detaches. 9. **DNA rewinds**, and **mRNA exits** the nucleus.

Question 30

Where does translation take place in the cell?

Answer 30

In the ribosome.

Question 31

What are the main steps of **translation**?

Answer 31

1. The ribosome attaches to the mRNA strand at the **start codon**. 2. A tRNA with **complementary anticodon** binds to the codon, bringing a **specific amino acid**. 3. A second tRNA binds to the next codon with its specific amino acid. 4. A **peptide bond** forms between the amino acids (uses ATP). 5. First tRNA **detaches** and is recycled. 6. Ribosome moves along mRNA, repeating the process. 7. Continues until a** stop codon** is reached. 8. Polypeptide is released to fold into a functional protein.

Question 32

At any point during translation, how many tRNA molecules can be attached to the ribosome?

Answer 32

Two tRNA molecules

Question 33

What is ATP and why is it important for cells?