Chapter 5 - DNA & Chromosomes

Griffith's Experiment (1928)

Discovered the 'transforming principle' using virulent (S) and non-virulent (R) Streptococcus pneumoniae in mice.

Avery, MacLeod, and McCarty Experiment (1944)

Demonstrated that DNA, not protein or RNA, is the chemical nature of genetic material.

Hershey and Chase Experiment (1952)

Used bacteriophages to confirm that DNA carries genetic information into host bacteria.

Components of a Nucleotide

A five-carbon sugar, a nitrogenous base, and a phosphate group.

Phosphodiester Bond

Covalent linkage connecting the 3′-hydroxyl of one sugar to the 5′-phosphate of the next.

DNA Strand Polarity

Determined by 5′ end (free phosphate) and 3′ end (free hydroxyl).

Purine vs Pyrimidine Bonding to Sugar

N9 of purines and N1 of pyrimidines bond to the 1′ carbon of ribose/deoxyribose.

Chargaff's First Rule

In DNA, the amount of adenine equals thymine (A=T) and guanine equals cytosine (G=C).

Chargaff's Second Rule

The percentage of G+C content is constant for a given species.

Rosalind Franklin's Contribution

Provided X-ray diffraction images (Photo 51) proving DNA is helical with stacked base pairs.

Hydrogen Bonds Between Base Pairs

Two hydrogen bonds between A and T; three hydrogen bonds between G and C.

B-DNA Structural Parameters

Right-handed helix, ~20 Å diameter, 10 base pairs per turn, 34 Å pitch.

Antiparallel Strands

The two DNA strands run in opposite 5′-to-3′ chemical directions.

Human Genome Size and Chromosomes

Approximately 3 billion base pairs packaged into 23 pairs of chromosomes.

Giemsa Stain Dark Bands

Indicate AT-rich regions of chromosomes.

Reciprocal Chromosomal Translocation

Swapping of segments between non-homologous chromosomes; commonly found in cancer cells.

Origins of Replication

Specialized DNA sequences where DNA replication begins bidirectionally during interphase.

Centromere Function

Attaches duplicated chromosomes to the mitotic spindle for proper segregation during M phase.

Telomeres

Repetitive DNA sequences at chromosome ends that ensure complete replication and protection.

Nucleolus Function

Nuclear site where ribosomal RNAs are synthesized and combined with proteins to form ribosomes.

Euchromatin

Extended, less condensed form of interphase chromatin containing actively expressed genes.

Heterochromatin

Highly condensed interphase chromatin containing mostly gene-poor regions.

Nucleosome Core Structure

DNA wrapped ~1.7 times in a left-handed coil around a histone octamer disc.

Histone Octamer Composition

Two molecules each of histones H2A, H2B, H3, and H4.

Chromatin Looped Domains

Chromatin loops clamped at the base by nonhistone chromosomal proteins.

Chromatin-Remodeling Complexes

ATP-dependent protein complexes that loosen and reposition nucleosomes along DNA.

Histone Tail Modifications

Covalent additions (acetyl, methyl, phosphate) to N-terminal tails regulating chromatin accessibility.

Histone Acetylation Effect

Neutralizes positive charge on lysines, loosening chromatin structure to promote gene expression.

Constitutive Heterochromatin

Permanently condensed, repetitive DNA found mainly at centromeres and telomeres.

Facultative Heterochromatin

Reversibly silenced chromatin regions regulated by mechanisms like histone deacetylation or RNAi.

Heterochromatin Protein 1 (HP1)

Binds specifically to methylated H3K9 to facilitate heterochromatin spreading and condensation.

X-Chromosome Inactivation

Random condensation of one X chromosome into heterochromatin in early female mammalian embryos.

Barr Body

The dense, inactive X chromosome observed at the nuclear periphery of female cells.