Chromosome Structure, DNA Packaging, and Repair Mechanisms

Centromere

Ensures proper segregation of chromosomes during cell division

Telomere

Protect chromosome ends and prevent loss of coding DNA during replication

Origin of replication

Sites where DNA replication begins

Nucleosome

DNA wrapped around a histone octamer (147 bp)

Histone H1

Tightens DNA packing by binding DNA as it exits the nucleosome

Euchromatin

Less condensed, transcriptionally active chromatin

Heterochromatin

Highly condensed, transcriptionally silent chromatin

Facultative heterochromatin

Variable by cell type/developmental stage; linked to gene silencing

Constitutive heterochromatin

Always condensed; found at centromeres and telomeres

ATP-dependent chromatin remodeling complexes

Regulates DNA accessibility

Barrier sequences

Stop heterochromatin from spreading into euchromatin

Calico cat fur patterns

Example of X-chromosome inactivation in real life

Epigenetic inheritance

Transmission of histone marks/modifications during DNA replication

Direction of DNA synthesis

5' → 3'

Proofreading

DNA polymerase's 3' → 5' exonuclease activity that corrects errors

Depurination

Loss of a purine base, leaving an empty sugar-phosphate backbone

Deamination

Removal of an amino group from cytosine, producing uracil

Thymine dimers

Mutation caused by UV light

Excision, re-synthesis, ligation

Three main steps of most DNA repair mechanisms

Mismatch repair

Repair mechanism that fixes replication errors that escape proofreading

Xeroderma pigmentosum

Disease caused by defective repair of thymine dimers

Non-homologous end joining (NHEJ)

Error-prone double-strand break repair by ligating flushed ends

Homologous recombination (HR)

Error-free repair using a sister chromatid as a template

Homologous recombination occurrence

After DNA replication, before sister chromatids separate

Mutations accumulation with age

DNA damage is not fully repaired over time

Genome sequences

Records replication fidelity

Non-homologous end joining (NHEJ) consequences

Can result in loss of genetic information

Homologous recombination (HR) characteristics

Generally error-free