Genetics 8/17

The primary structure of DNA and RNA

• Macromolecule

• Polymer: chain of many nucleotide units linked together

• Made up of 3 units:

  • Pentose sugar

  • Phosphate group

  • Nitrogenous base

Pentose sugar in DNA

2-deoxyribose (H on 2’ carbon)

Pentose sugar in RNA

Simple ribose (hydroxyl group [OH] in 2’ carbon)

Why is RNA less stable than DNA?

RNA is less stable than DNA primarily because of its 2′-hydroxyl group on the ribose sugar, which makes it prone to chemical self-cleavage, and its single-stranded structure, which leaves it more exposed to environmental degradation.

The phosphate group in DNA and RNA…

• Carries a negative charge

• Is always bonded to the 5’ carbon atom of the sugar

What is a nucleotide made up of?

Nitrogenous base, a phosphate group and a sugar (deoxyribose in DNA or ribose in RNA)

What are the 2 nitrogenous bases (Purines) in DNA?

Adenine and Guanine

What are the 3 nitrogenous bases (Pyrimidines) in RNA?

Cytosine, Thymine, and Uracil

You are studying a small eukaryotic gene of about 2kb in length. How many nucleotides are there in this gene?

4,000

What is the A-T and G-C base pairing ratio?

Both 1; A+T = G+C

What is important about the polynucleotide strands of DNA?

• The two strands are antiparallel 5'  3' and 3' 5’

• Phosphodiester bonds are between 5'-PO4 and 3'-OH groups of adjoining nucleotides on each strand

• Hydrogen bonds are between complementary bases of opposite strands (intermolecular)

  • A-T base-pairs have two H-bonds

  • G-C base-pairs have three H-bonds

What is a hydrogen bond?

A weak electrostatic interaction between a hydrogen atom covalently bound to an electronegative atom (e.g. N and O) and another electronegative atom.

What is hybridization? What makes this possible?

The process where two single-stranded DNA (or RNA) molecules bond together via complementary base pairing to form a double-stranded hybrid. Weak hydrogen bonds

Why is more energy required to denature G-C base pairs than A-T base pairs?

G-C pairs form three hydrogen bonds, while A-T pairs form only two

What is the secondary structure of ssDNA and RNA?

• Intramolecular hydrogen bonding: complementary base-pairing within a molecule of DNA or RN

  • Hairpin vs Stem

What is the secondary structure of DNA?

Double helix structure

• The two strands are antiparallel and held together by hydrogen bonds (H bonds) between complementary base

  • Sugar – phosphate backbone is on the outside

  • Base pairs are in the middle

What is the tertiary structure of BACTERIAL DNA?

Supercoiling

What are topoisomerases?

Add or remove rotations in DNA by breaking and then rejoining DNA strands

What is the tertiary structure of EUKARYOTIC DNA?

Linear chromosomes

• They also include arrays of proteins, together they are known as chromatins

• Protein components of chromosomes are determine function: condensation, segregation, and organization of chromosomes during mitosis and meiosis

True or False:

Each chromosome is approximately ½ DNA and ½ protein

True

Define sexu@l reproduction.

The mixing of genomes via meiosis and fusion of gametes

Define dioecious species.

Either male or female reproductive structures are present in each

individual.

Define monoecious species.

Both male and female reproductive structures are present in each

individual.

XX-XY sex-determining system:

• XX = Female

• XY = Male

ZZ-ZW sex-determining system:

• ZZ = Males

• ZW = Females

• Used by birds, butterflies, some reptiles, and some amphibians