A1.2 NUCLEIC ACIDS

Define DNA

Define DNA

• Deoxyribo nucleic acid

• Genetic / hereditary material for all living organisms — passed to offspring

• Found in nucleus, mitochondria & chloroplast of eukaryotes, and plasmid & nucleoid of prokaryotes

• Genetic code is contained within the nucleic acid

Outline the two types of nucleic acids

DNA & RNA

• some viruses use RNA as genetic material, but viruses are not considered to be living

• HIV: retrovirus with RNA

Outline the base and genetic code of nucleic acids

• The sequence of nitrogenous base pairs/nucleotides in each nucleic acid from the basis of the genetic code

• Bases in DNA:

  • Adenine and thymine

  • Guanine and cytosine

• Bases in RNA

  • Adenine and uracil

  • Guanine and cytosine

• The genetic code is universal to all organisms with three bases pair sequences called codons, andwhich correspond to the sameaminoacid in nearly all organisms

Describe the components of nucleotide, and the formation of the nucleic acids DNA and RNA

NUCLEOTIDES:

1. Pentose 5-C sugar (either ribose RNA OR deoxyribose DNA) — DNA & RNA are polymers of nucleotides

2. Phosphate group

3. Nitrogenous base

STRUCTURE & FORMATION OF DNA/RNA:

• Made through condensation reactions to join nucleotides to a growing chain → forms phosphodiester bond (and water)

• New nucleotides are added to the 3’ carbon of the sugar in the 5’ to 3’ direction

• Sugar: phosphate backbone on the outside

• Nitrogenous bases are on the inside

Outline the structure of DNA

1. Double stranded molecule → forms doublehelix that wraps around each other regularly

2. Nucleotides are the subunits, and DNA is a polymer of nucleotides

3. Hydrogenbonds from between complementary base pairs (adenine & thymine, cytosine & guanine) and covalentbonds form between phosphate group and sugar

4. The sugar-phosphate backbone is on the outside (of dna and rna)

5. Strands are anti-parallel (5’ - 3’ one way, 3’ to 5’ other way) → linear strands of DNA are formed in eukaryotes and circular DNA is found in prokaryotes

Draw the structure & similarities of the nucleotides DNA and RNA

Compare and contrast DNA and RNA

Outline the role of complementary base pairing in the replication of genetic information and expression

1. Complementary base pairing between nitrogenous bases allows for genetic information to be replicated (DNA replication) and expressed (transcription of DNA to form mRNA and translation of protein synthesis using mRNA strand)

2. Hydrogen bonds form between complementary base pairs: Adenine pairs with thymine (DNA) or uracil (RNA)/ cytosine pairs with guanine

3. REPLICATION: DNA polymerase III adds new DNA nucleotides (polymerise, elongate) to the growing stand between the old template and the new strand

   • Adds in the 5’ to 3’ direction to the 3’ end

   • Adds nucleotides based on complementary base pairing

   • Occurs when the cell divides, in the s-phase (interphase) of the cell cycle

   • Produces identical copies of DNA

4. TRANSCRIPTION: RNA polymerase adds new RNA nucleotides to the growing mRNA stand based on the pairing between the template/antisense strand of DNA and mRNA

   • Adds in the 5’ to 3’ direction to the 3’ end

   • Adds nucleotides based on complementary base pairing

   • Produces mRNA from DNA

5. TRANSLATION: Complementary base pairing between codons in mRNA and anticodons tRNA

   • Produces proteins from mRNA strand

   • Ribosome moves in the 5’ to 3’ direction of mRNA

   • Information in mRNA is used to bring amino acids & bring them together to form a polypeptide/proteins

Outline the diversity of possible DNA base sequences and the limitless capacity of DNA for information storage

1. DNA is an information storage molecule

2. DNA is a molecule of hereditary: information is passed to offspring — in DNA replication, info is stored in DNA and passed from cell to cell

3. Information is translated into protein through synthesis — a sequence of nitrogenous bases form a code that stores information used to make proteins

4. Diversity by any length of DNA and any base sequence/order:

   • Four bases: adenine, thymine, cytosine, guanine

   • Possibilities are 4^n, where n = length of DNA

Outline the conservation of genetic code across all life forms as evidence of universal common ancestry

1. All living organisms have DNA as the molecule of hereditary

2. The genetic information is conserved in the genetic code of all living organisms

3. The genetic code is universal and evidence for a universal common ancestor (LUCA)

Outline the purine-to-pyrimidine bonding as a component of DNA helix stability

1. There are two types of nucleotides: purine and pyrimidines

2. Purines form hydrogen bonds with pyrimidines by complementary base pairing

3. Adenine forms 2 hydrogen bonds with thymine

4. Guanine forms 3 hydrogen bonds with cytosnie

5. Purine to pyrimidine (hydrogen bonding) allows for DNA helix stability → maintains consistent width

   • purine + purine → too wide

   • pyrimidines + pyrimidines → too narrow

Describe the structure & function of a nucleosome

1. Nucleosomes are structural units that organise DNA

2. Nucleosomes condense the size of DNA by 8,000x

3. Nucleosomes are 8 histone proteins and a linker histone protein that connects nucleosomes together

4. DNA is wrapped twice around the nucleosomes

5. The positively charged histone proteins electro-statically bind to the negatively charged phosphates in DNA

Discuss the Hershey-Chase experiment

1. Wanted to determine if protein or DNA was the molecule of heredity

2. Viruses/bacteriophages were labelled with radioactive isotopes → infect cells and inject their genetic material

   • Radioactive phosphorus was incorporated into DNA

   • Radioactive sulphur was incorporated into proteins

3. Centrifugation separates the supernatant (phage) from the pellet (bacteria)

4. Radioactive labelled protein (coat/capsid) remains in the supernatant → protein isn’t injected as the hereditary molecule

5. Radioactively labelled DNA is found in the pellet → entered the cell → DNA is responsible for hereditary

Discuss Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms

1. Erwin Chargraff analysed the chemical composition of A,T,C,and G bases from DNA of different species

2. Bases were not in equal quantities

3. The number of bases differed between the species, but not within a species → contributed to the idea that DNA is genetic material

4. The amount of A=T and the amount of C=G (the purine to pyrimidine ratio equals 1)