A1.2

What is the genetic material of living organisms?

DNA (deoxyribonucleic acid) carries the genetic code in all living organisms and is mainly found in the nucleus, mitochondria, and chloroplasts.

What is RNA and its main function?

RNA (ribonucleic acid) is a nucleic acid involved in protein synthesis and is the main component of ribosomes.

Do all organisms use DNA as genetic material?

No, some viruses such as SARS-CoV-2, Ebola, and influenza use RNA as their genetic material.

Why are viruses not considered living organisms?

They cannot replicate independently and lack cellular structure; they depend on host cells for reproduction.

What are nucleotides?

The building blocks of nucleic acids, consisting of a phosphate group, a pentose sugar, and a nitrogenous base.

What are the components of a nucleotide?

A phosphate group, a pentose sugar (deoxyribose or ribose), and a nitrogen-containing base.

What are the nitrogenous bases in DNA?

Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

Which base replaces thymine in RNA?

Uracil (U) replaces thymine in RNA.

What are purines and pyrimidines?

Purines (A and G) have two rings; pyrimidines (C, T, U) have one ring.

How are nucleotides joined together?

Through condensation reactions forming covalent phosphodiester bonds between the phosphate of one nucleotide and the sugar of another.

What is released when nucleotides bond?

A molecule of water (H₂O) is released in each condensation reaction.

What is the sugar-phosphate backbone?

A repeating chain of alternating sugars and phosphates in DNA or RNA strands.

How does RNA differ from DNA in structure?

RNA is usually single-stranded, shorter, and contains ribose sugar and uracil instead of thymine.

What are the three main types of RNA?

mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

What is the function of mRNA?

It carries genetic information from DNA in the nucleus to ribosomes for protein synthesis.

What is the function of tRNA?

It carries amino acids to the ribosome during translation.

What is the function of rRNA?

It forms part of ribosomes, where protein synthesis occurs.

What is the structure of DNA?

A double helix made of two antiparallel strands held together by hydrogen bonds between complementary bases.

What are antiparallel strands?

DNA strands running in opposite directions: one from 5' to 3', the other from 3' to 5'.

What is complementary base pairing?

A pairs with T (2 hydrogen bonds), and C pairs with G (3 hydrogen bonds).

Why is complementary base pairing important?

It ensures accurate DNA replication and transcription.

What maintains the shape of the DNA double helix?

Hydrogen bonds between bases and base stacking between purine-pyrimidine pairs.

What is the genetic code?

The sequence of bases in DNA that determines the order of amino acids in a protein.

What is a codon?

A sequence of three DNA bases that codes for one amino acid.

Why is the genetic code called universal?

Because almost all organisms use the same codons to code for the same amino acids.

Why does the universality of the genetic code support evolution?

It suggests that all organisms share a common ancestor.

What are conserved sequences?

Base sequences that remain unchanged across species, indicating essential functions such as transcription or translation.

How do DNA and RNA differ in base composition?

DNA contains thymine; RNA contains uracil instead.

How do DNA and RNA differ in sugar?

DNA contains deoxyribose; RNA contains ribose.

How many strands does DNA have compared to RNA?

DNA is double-stranded; RNA is single-stranded.

What is directionality in DNA and RNA strands?

Each strand has a 3' end (hydroxyl group) and a 5' end (phosphate group).

Why is directionality important?

It determines how enzymes read and replicate genetic information during transcription and translation.

What does 5' to 3' mean?

It refers to the direction in which new nucleotides are added to a growing strand during replication or transcription.

What is the coding strand?

The DNA strand with the base sequence that corresponds to the mRNA produced (except T is replaced by U).

What is the template strand?

The DNA strand used by enzymes to synthesize the complementary mRNA strand.

Who discovered the double helix structure of DNA?

James Watson and Francis Crick in 1953.

What did Chargaff's data show?

That the amount of adenine equals thymine, and guanine equals cytosine (A=T, G=C).

What did Chargaff's findings disprove?

The tetranucleotide hypothesis proposed by Levene, which suggested DNA had a simple repeating unit.

What are purines and pyrimidines' structural differences?

Purines (A, G) have two rings; pyrimidines (C, T) have one, ensuring uniform helix width.

What are nucleosomes?

Units of DNA packaging in eukaryotic cells consisting of DNA wrapped around histone proteins.

What proteins are involved in forming nucleosomes?

Histone proteins; eight form the core (histone octamer) with DNA coiled around it twice.

What is linker DNA?

The DNA segment connecting adjacent nucleosomes.

What are the functions of nucleosomes?

They help supercoil DNA to fit in the nucleus, protect it from damage, and regulate transcription.

What is supercoiling?

The over- or under-winding of DNA that allows long molecules to fit inside the nucleus compactly.

What is the relationship between histones and gene expression?

Chemical modifications of histones can promote or suppress transcription.

What is molecular visualisation software used for?

To view and analyze molecular structures like proteins and DNA in 3D.

What is an example of molecular visualisation software?

Mol* or JSmol, available via the Protein Data Bank (PDB).

What structure can be viewed in the PDB under code 6T79?

The 3D structure of the human nucleosome.

What did the Hershey-Chase experiment demonstrate?

That DNA, not protein, is the genetic material responsible for heredity.

How did Hershey and Chase label DNA and protein?

They used radioactive phosphorus-32 to label DNA and sulfur-35 to label proteins.

What organism did Hershey and Chase use?

A bacteriophage virus that infects E. coli bacteria.

How did they show DNA was the genetic material?

Only bacteria infected with ³²P-labelled phages (DNA) became radioactive, proving DNA entered cells.

Why were radioisotopes essential for the Hershey-Chase experiment?

They enabled detection of which molecule entered the cell, DNA or protein.

What is the problem of induction in science?

It's impossible to prove a hypothesis absolutely true; data can only support or falsify it.

Who emphasized falsification in scientific progress?

Philosopher Karl Popper.

What was the tetranucleotide hypothesis?

Levene's idea that DNA was composed of a repeating four-base unit, later disproven by Chargaff.

How was Levene's hypothesis falsified?

Chargaff's data showed variation in base composition across species, inconsistent with a simple repeating pattern.

What is the significance of Chargaff's rules for DNA structure?

They supported the idea of complementary base pairing, leading to Watson and Crick's model.