Biology 4.3 and 4.4

4.3 DNA Structure and Replication
4.4 RNA

What You Will Learn

• Structure and function of DNA

• Chargaff's Rule

• DNA replication and DNA polymerase

What Does DNA Look Like?

• DNA resembles a twisted ladder.

  • Steps = Nitrogen Bases (A, T, G, C)

  • Railings = Sugar-phosphate backbone

• Scientists use models to study DNA’s complex structure.

DNA Structure

• DNA is made of nucleotides, each with:

  • Backbone

    • A sugar (deoxyribose)

    • A phosphate group

  • A nitrogen base (A, T, G, or C)

• The double helix shape was discovered by Watson and Crick with contributions from Rosalind Franklin using X-ray crystallography.

Multiple Choice

The backbone of DNA is made of

• base pairs

• sugars and phosphates

• only sugars

• only phosphates

Chargaff's Rule

• Discovered by Erwin Chargaff:

  • Amount of A ≈ Amount of T

  • Amount of G ≈ Amount of C

• This discovery helped understand DNA’s complementary base pairing

Multiple Choice

If a DNA molecule has 30% Guanine bases, how many Cytosine bases will it have?

• 30%

• 25%

• 20%

• 10%

Base Pairing and the Double Helix

• Complementary Base Pairing:

  • A bonds with T (2 hydrogen bonds)

  • G bonds with C (3 hydrogen bonds)

• Keeps the helix uniform in shape.

• Steps of the ladder = Base pairs (A-T, G-C)

Multiple Choice

Adenine pairs with

• Thymine

• Thyme

• Guanine

• Glutamine

DNA Replication

• Occurs in the S phase of the cell cycle.

• Key Steps:

  1. DNA helicase separates strands by breaking hydrogen bonds.

  2. DNA polymerase builds new complementary strands.

• Semi-conservative process:

  • Each new DNA molecule = 1 parent strand + 1 new strand.

Multiple Choice

The first step in DNA replication is

• to unwind and unzip the two strands

• to break the backbone in half so the sequence can be copies

• to attach new nucleotides

• to proof read the sequence

Summary

• Chargaff’s Rule: A ≈ T, G ≈ C

• DNA has a double helix structure with complementary base pairs.

• Replication: DNA is copied semi-conservatively.

• DNA polymerase ensures accuracy.

What You Will Learn

• How RNA differs from DNA

• Types of RNA and their functions

• Role of RNA in protein synthesis

How Information Moves

• DNA is in the nucleus, proteins are made in the cytoplasm.

• RNA acts as the messenger moving genetic information.

• Central Dogma: DNA → RNA → Protein

RNA vs. DNA

• RNA (Ribonucleic Acid):

  • Works alongside DNA to produce proteins.

  • Carries instructions from DNA to ribosomes.

• RNA differs from DNA:

  • Single-stranded (DNA: double-stranded)

  • Uracil (U) replaces thymine (T)

  • Contains ribose sugar (DNA: deoxyribose)

Multiple Choice

What sugar does RNA have in its backbone?

• Ribose

• Deoxyribose

• Glucose

• Fructose

RNA and Protein Synthesis

• Protein synthesis requires all three types of RNA:

  1. mRNA: Provides the genetic code.

  2. tRNA: Transfers amino acids to the ribosome.

  3. rRNA: Facilitates the assembly of proteins.

Multiple Choice

Which molecule exits the nucleus after being made?

• RNA

• DNA

• Water

• Energy

Summary

• RNA is single-stranded, contains ribose, and uses uracil.

• Types of RNA:

  • mRNA: Carries genetic instructions.

  • rRNA: Forms ribosomes.

  • tRNA: Transfers amino acids.

• RNA is essential for protein synthesis.