DNA Organization, Replication,& Repair

Model for the structure of the nucleosome

• Higher-order structures provide for the compaction of chromatin– 10-nm fibril – 30-nm chromatin fiber – Loops or domains• Interphase chromosomes

• Some regions of chromatin are “active” & others are “inactive”

• Generally, every cell of an individual metazoan organism contains the same genetic information

• Sensitive to digestion by a nuclease • In many cases, it seems that if a gene is capable

of being transcribed, it very often has a DNase-hypersensitive site(s) in the chromatin immediately upstream.

• Heterochromatin – Transcriptionally inactive chromatin– Densely packed

• Euchromatin – Transcriptionally active chromatin

DNA is organized into chromosome

• Telomeres – Consist of short, repeat TG-rich sequences

• Each sister chromatid contains one double-stranded DNA molecule.

• During interphase, the packing of the DNA molecule is less dense than it is in the condensed chromosome during metaphase.

• Metaphase chromosomes are nearly completely transcriptionally inactive.

DNA is organized into chromosome

• The human haploid genome – 3 *109 bp– 23 chromatids – Each sister chromatid contains one double-

stranded DNA molecule. • Each chromatid would contain one double-

stranded DNA molecule.

extent of DNA packaging in metaphase chromosomes

Coding Regions Are Often Interruptedby Intervening Sequences

• Protein coding regions of DNA– mRNA – exons

• Nonprotein coding DNA– Introns

• 1-50

– hnRNA • Primary transcript

– The function• is not clear

Much of the mammalian genome is redundant & much is not transcribed

• Sequence classes– Unique sequence, or nonrepetitive, DNA – Repetitive sequence DNA.• Centromeres & telomeres• Transcriptionally inactive & may play a structural role

• Different combinations of genes are expressed in each tissue

• In human DNA, at least 30% of the genome consists of repetitive sequences