A1.2: Nucleic Acids

What is a nucleic acid and how is it made?

A nucleic acid is a biological polymer made up of repeating monomers of nucleotides covalently bonded together through a condensation reaction called polymerisation.

2 types of nucleic acids

Deoxyribonucleic acid and Ribonucleic acid

Function of DNA

DNA is the universal genetic code for all living organisms. It consists of sections called genes that code for proteins that give us our traits. DNA preserves genetic information across generations, which gives us our traits.

DNA as the genetic material for all living things

Viruses are not considered to be alive because they can only reproduce in host cells, some viruses consists of RNA instead of DNA.

Components of Nucleotides

A nucleotide consists of a 5 carbon pentose sugar, a phosphate group which is a functional group consisting of phosphate and oxygen, and a nitrogenous base which is a molecule consisting of a nitrogen and it acts as a base

How is the sugar-phosphate backbone formed?

The phosphate group attached to the 5’ carbon of a pentose sugar forms a phosphodiester covalent bond with a hydroxyl group of a 3’ carbon of another pentose sugar through a condensation reaction called polymerisation.

Direction and growth of sugar phosphate chain

From 5’ end to 3’ end

What forms the genetic code?

The sequence of nitrogenous bases in the DNA form the genetic code

5 nitrogenous bases and their classifications

Purines: Adenine, Guanine

Pyrimidines: Cytosine, Thymine, Uracil

Difference between Pyrimidines and Purines

Purines have a double-ring structure, and pyrimidines have a single ring structure

Nitrogenous base pairing rules

A-T, A-U, G-C

4 Differences between RNA and DNA

• RNA is single stranded, DNA is double-stranded

• DNA has deoxyribose sugar, RNA has ribose sugar

• DNA has thymine, RNA has uracil

• RNA pentose sugar has a hydroxyl group attached at its 2’ C, DNA pentose sugar has a Hydrogen atom attached to its 2’ C

Formation of RNA molecule

RNA nucleotides are covalently bonded together to form the polymer of RNA

DNA double-helix structure

DNA nucleotides are covalently bonded together to form a DNA strand, 2 strands are joined together by complimentary base pairing, and they run in antiparallel motion. The DNA is twisted to form its helical shape.

How do nitrogenous base pairs bind to each other

Nitrogenous base pairs bind to each other through hydrogen bonding

DNA replication

DNA helicase splits the DNA strand by breaking the hydrogen bonds, and a DNA strand is used as a template to create a new one, preserving the DNA sequence

What is gene expression?

The process by which genetic code in the DNA is translated into proteins.

Importance of complimentarity in gene expression

It ensures that the same protein is produced every time the gene is expressed, which maintains the characteristics of the cell and the organism.

Diversity of genetic code

Any base sequence is possible at any length of DNA allowing for diversity

Conservation of genetic code and common ancestry

All living organisms use the same genetic code providing evidence for common ancestry

Central dogma and directionality

DNA is replicated and is used as a template to transcribe RNA which is then translated by ribosomes to produce proteins. These processes occur in the 5’ to 3’ direction

Number of hydrogen bonds between A-T and C-G

A-T have 2 hydrogen bonds, C-G have 3 hydrogen bonds

How is DNA organised in a prokaryote cell?

DNA found in the nucleoid region in the plasma, referred to as “Naked DNA”

What is a nucleosome?

It consists of a length of DNA of around 150 base pairs wrapped around a core of 8 histone proteins and a special histone H1 linker protein.

DNA linker

A section on the DNA where the DNA strand from one nucleosome flows into another.

Relationship between nucleosomes and chromosomes

DNA is coiled around multiple nucleosomes, which together coil to form the chromatin fibre which then coil to form chromosomes.

Hershey-Chase experiment

Viruses called bacteriophages infected bacteria. They labeled the virus's outer protein coat with radioactive sulfur and its inner DNA with radioactive phosphorus. After spinning the infected bacteria in a blender, they found that only the radioactive DNA entered the bacterial pellet, while the proteins stayed outside in the supernatant.

Chargaff’s experiment

Chargaff used chromatography to falsify the tetranucleotide hypothesis and found that A = T and C = G

What is complementarity based on?

Complementary base pairing depends on the arrangement of hydrogen bonds between the bases

Diversity of DNA

Any length of DNA and any base sequence is possible; DNA has enormous capacity for storing data.