Quiz 3

DNA and Chromosomes

Overview

• This chapter covers the structure of DNA, how DNA functions as the blueprint, the structure of eukaryotic chromosomes, and the replication and condensation of DNA.
• DNA molecules become chromosomes only after DNA condensation.
• Chromosomes are visible only when cells are dividing.

Historical Review: Genes are Made of DNA

Griffith’s Experiment (1928)

• Demonstrated that genes are made of DNA.
• When bacteria are killed, their cells burst, releasing their contents.
• Something among the released contents transforms another strain of bacteria.

Avery, MacLeod, and McCarty’s Experiment (1937-40)

• Separated the components of dead bacteria.
• Combined each component with the R strain of bacteria to see which one transformed the R strain.
• Enzymes were used to digest components, leaving only one component each time to identify the transforming agent.

Double-Stranded DNA Molecule

• Long strands of DNA pair up to form a double-stranded DNA molecule.
• Each strand is made of subunits called nucleotides.
• Every nucleotide is composed of three parts: ribose, phosphate, and a base.
• Hydrogen bonds connect the two strands together.

Nucleotide Structure – Ribose

• Ribose is used for RNA.
• Deoxyribose is used for DNA.

Deoxyribonucleic Acid (DNA)

• Composed of two strands of molecules.
• Hydrogen bonds connect the two strands.
• Each strand is composed of a backbone and bases.
• The backbone is made of phosphates and a 5-carbon sugar called deoxyribose.
• The backbone in RNA is ribose.
• Four different types of bases attach to the #1 carbon.
• A phospho-sugar + a base becomes a nucleotide.
• Based on the different bases attached to the #1 carbon, there are 4 different types of nucleotides.

Nucleotide Structure – Bases: Pyrimidines and Purines

• Four different types of bases are used on the #1 carbon to make 4 different nucleotides.
• Adenine (A), Guanine (G), Cytosine (C), and Thymine (T) are the bases in DNA.
• In RNA, Uracil (U) replaces Thymine (T).
• This is the 2nd difference between DNA and RNA.

Nucleotide Structure – Phosphate and Bases

• Phosphate is associated with energy.
• Two of the tri-phosphates will break off and release energy for the nucleotides to bind.
• New chemical bonds are formed by using the energy released from breaking off the two phosphates.

Nucleotide Structure – Nucleic Acids

• When nucleotides connect together, a condensation reaction occurs.
• Two nucleotides connect and lose a H_2O molecule.
• The new covalent bond formed is called a phosphodiester bond.
• This process is similar to other processes that build up macromolecules, but other macromolecules might not have phosphate.
• With the combination of 4 different nucleotides, a long strand of code is created.

Double-Stranded DNA Molecule

• Long strands of DNA pair up to make a double-stranded DNA molecule.
• Hydrogen bonds connect the two strands.
• Complementary base pairs compose the antiparallel double helix.
• Double helix structure is the result of this pairing.

How Chemicals Carry Message

Sequences Encode Messages

• Languages and messages are symbols arranged according to certain rules.
• Messages can be the blueprint for building something.

Gene

• Sections of DNA molecules code for proteins.
• Not all DNA encodes genes; there are many non-coding regions.
• Non-coding regions may serve other functions or protect coding regions.
• When extreme environmental factors occur, they are more likely to affect the non-coding region.
• A gene is a section of DNA molecules that codes for one protein.
• Proteins perform functions.

DNA Replication and Condensation

• Chromosomes only exist during cell division.
• DNA replication is necessary before Mitosis or Meiosis and occurs in multiple locations.
• Once DNA replication is completed, the replicated DNA condenses into rod-shaped chromosomes for distribution to daughter cells.
• After cell division, the chromosomes loosen up and become DNA again.

Histones During DNA Condensation

• For condensation, long strands of DNA molecules wrap around histones.
• Each DNA-histone complex is called a nucleosome.
• These DNA-histone complexes further fold into chromosomes.

Chromatin and Chromosomes

• The structure of the nucleosomes consists of DNA wrapped around histones.

Nucleosomes

• Nucleosomes can be seen under an electron microscope, most likely in interphase.
• This marks the beginning of condensation.

Chromosome vs. Chromatids

• Condensed duplicated chromosomes are made of two identical chromatids attached together.