2.3 - Nucleotides and nucleic acids

DNA vs RNA

• Both carry information

• Both polymers of nucleotides

• DNA holds genetic information

• RNA transfers genetic information form DNA to ribosomes

What are ribosomes made of?

RNA and proteins

A nucleotide consists of:

Pentose (5-carbon sugar), a nitrogenous organic base and a phosphate group.

Difference in DNA and RNA

‘Deoxy’ribose (sugar) - contains one less oxygen atom in comparison to ribose (sugar)

Bonds joining nucleotides together

Phosphodiester bonds

Formed by a condensation reaction between a hydroxyl group of the sugar and the hydroxyl group of the phosphate group of two different nucleotides.

A DNA molecule:

• Has a double helix structure,

• Created between two polynucleotides joined together by hydrogen bonds that form between the complementary bases.

A molecule of RNA:

• In comparison to DNA it is a short, single polynucleotide chain.

ATP

• Adenosine triphosphate

• It is a nucleotide derivative

• Consists of:

  • Ribose (sugar)

  • Adenine (base)

  • Three phosphate groups

Hydrolysis of ATP

• Energy is released

• ADP is formed alongside a phosphate molecule

• Process is catalysed by ATP hydrolase (an enzyme)

Ribose, adenine, phosphate groups

• Can make:

  • Adenosine (just adenine and ribose)

  • Adenosine monophosphate (adenine, ribose and one phosphate group)

  • Adenosine diphosphate - ADP (adenine, ribose and two phosphate groups)

  • Adenosine triphosphate - ATP (adenine, ribose and three phosphate groups)

Use of inorganic phosphate

Used to phosphorylate other compounds, making them more reactive in result.

How is ATP made

• Through the condensation of ADP and an inorganic phosphate catalysed by ATP synthase (an enzyme).

• This occurs during photosynthesis and respiration.

Semi-conservative replication

• Ensures genetic continuity between generations of cells,.

• It is a method where DNA replicates, creating two molecules of DNA that consist of one original DNA strand and one newly synthesised DNA strand.

DNA replication steps:

• DNA helicase unwinds and unzips the DNA (breaking hydrogen bonds between complimentary base pairs. Separating the two polynucleotide chains.

• Both strands are used as templates and complimentary base pairing occurs between each template strands and free nucleotides.

• Adjacent nucleotides are joined by phosphodiester bonds formed in a condensation reaction (done by DNA polymerase).

What is the genetic code?

It is the order of bases on DNA.

Consists of triplets of bases.

A triplet of bases

• Known as a codon

• Codes for a specific amino acid

Peptide bond

• Bond that joins amino acids together

• Forming a polypeptide chain

A gene

A sequence of bases on a DNA molecule coding for a sequence of amino acids that form a polypeptide chain.

Non-coding sections of DNA

Introns

Coding regions of DNA

Exons

The genetic code is non-overlapping…

• Each triplet is read once and triplets don’t share any bases.

Degenerate

• More than one triplet codes for the same amino acid.

• Reduces the phenotypic effect (observable characteristics resulting from a mutation) of mutations (mistakes in the base sequence).

Three types of mutations

• Base deletion

• Base inspersion

• Base substitution

What could a change in the base sequence of DNA do.

• Alter the sequence of amino acids

• Therefore alter the protein.

Examples of harmful mutations.

• A mutation that leads to the production of sticky mucus - causes cystic fibrosis

• Sickle cell anaemia - a mutated form of haemoglobin distorts the shape of red blood cells.

Start and stop codons

• In the genetic code

• Start and stop protein synthesis

What are the two stages of protein synthesis?

• Transcription

• Translation

Enzymes and molecules involved in transcription:

• One DNA strand (template/antisense)

• A molecule of mRNA

• DNA gyrase (unwinds)

• DNA helicase (breaks H bonds)

• Free nucleotides

• RNA polymerase

Process of transcription:

• DNA gyrase unwinds the double helix

• DNA helicase breaks the hydrogen bonds between the complementary base pairs. The DNA separates into its two polynucleotide strands.

• One DNA strand is used as the template (antisense) strand; free nucleotides line up along the template strand by complementary base pairing. RNA polymerase joins adjacent nucleotides through phosphodiester bonds.

• A single-stranded molecule of mRNA is formed.

• The mRNA moves out of the nucleus through the nuclear pores…

Enzymes and molecules involved in translation:

• A molecule of mRNA

• tRNA

• Ribosomes

• Amino acids

• A polypeptide chain

Process of translation:

• mRNA attaches to a ribosome (in the cytoplasm or on a rough endoplasmic reticulum).

• tRNA collects an amino acid from the cytoplasm and carries it to a ribosome.

• tRNA attaches itself to mRNA by complimentary base pairing, forming temporary hydrogen bonds between the codon (on mRNA) and anticodon (on tRNA). Two at a time.

• The amino acids that are attached to the tRNA molecules are joined by peptide bonds.

• tRNA molecules detach from the amino acids, leaving them behind.

• This is repeated, forming a polypeptide chain until a stop codon is reached on the mRNA and protein synthesis has ended.

• The tRNA picks up another amino acid in the cytoplasm and comes back, continuing the chain if another codon codes for this specific amino acid.

tRNA

• Is a single stranded molecule

• Has a binding site at one end - so it can only carry one type of amino acid.

• Has a triplet of bases at the other end - the triplet of bases is the anticodon complimentary to a mRNA codon and the amino acid.