1. DNA Organization, Replication, & Repair

2. Model for the structure of the nucleosome

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

4. 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.

5. Heterochromatin – Transcriptionally inactive chromatin – Densely packed Euchromatin – Transcriptionally active chromatin

6. 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.

7. DNA is organized into chromosome The human haploid genome – 3 *10 9 bp – 23 chromatids – Each sister chromatid contains one double- stranded DNA molecule. Each chromatid would contain one double- stranded DNA molecule.

9. extent of DNA packaging in metaphase chromosomes

12. Coding Regions Are Often Interrupted by Intervening Sequences Protein coding regions of DNA – mRNA – exons Nonprotein coding DNA – Introns 1-50 – hnRNA Primary transcript – The function is not clear

14. 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