1. Topic 7: The Organization and Control of Eukaryotic Genomes

2. Chromatin complex of DNA and proteins Nucleosome
”beads on a string”; basic unit of DNA packing
Consists of DNA wrapped around 8 histone proteins and held together by another
•histone protein (positively charged amino acids
phosphates of DNA are negatively charged)
Help to supercoil chromosomes and regulate transcription.
Animation of organization of DNA with histones (detailed)
Heterochromatin
•highly condensed interphase DNA (can not be transcribed)
Euchromatin •less compacted interphase DNA (can be transcribed)

3. Skill:
Utilization of molecular visualization software to analyse the association between protein and DNA within a nucleosome.
See page 348 of IB text and follow instructions:

4. Non-coding and Repetitive DNA
Some regions of DNA do not code for proteins (used to be called “junk DNA”
This DNA has other important functions…
Gene expression regulation
Introns
Telomeres : protect DNA and prevent degradation at ends.
Code for tRNA molecules
Repetitive sequences
Ex. GTTAC….
Ex. tandem repeats used in DNA profiling (these are repeated a different # of times in different individuals)
About 97%*of human DNA *Campbell and Reece estimate
Why telomeres are necessary
Some is repetitive DNA
Nucleotide sequences that are repeated many times in a genome
May help organize chromatin into chromosomes (ex. High concentration of repetitive DNA in centromere regions)

5. Theory of Knowledge Discussion
Highly repetitive sequences were once classified as “junk DNA” showing a degree of confidence that it had no role. To what extent do the labels and categories used in the pursuit of knowledge affect the knowledge we obtain?

6. Control of Gene Expression
Nucleosomes help regulate transcription
Gene expression is regulated by proteins that bind to specific base sequences in DNA. (see examples below)
Epigenetics Video
Why Necessary?
Cell Differentiation (ex. Stem cells become specialized)
Cells only express a fraction of their genes at one time. (3-5% on avg. in humans)

7. Control of Gene Expression
Methylation affects gene expression by preventing transcription
Turns genes off

8. Control of Gene Expression
The environment of a cell and of an organism has an impact on gene expression
Epigenetics Video

9. Nature of science: Looking for patterns, trends and discrepancies—
There is mounting evidence that the environment can trigger heritable changes in epigenetic factors.

10. End of IB Stuff

11. Methods of control Chromatin modification Regulation of Transcription
DNA packaging (heterochromatin vs. euchromatin
DNA methylation--attachment of methyl groups (-CH3) to DNA bases turns off DNA (prevents transcription)
Histone acetylation– attaching acetyl groups to histones– turns on DNA by loosening histone attachment
Regulation of Transcription
Transcription factors bind to DNA and stimulate transcription
Post-transcriptional regulation (alternative mRNA splicing, control of translation and protein processing to make a functional protein)

12. Molecular Biology of Cancer
Mutations in genes that control cell cycle.
Oncogene--•cancer-causing genes
Proto-oncogene •normal cellular genes involved in cell growth and division. May convert to oncogene
How? movement of gene; chromosome fragments that have rejoined incorrectly
2-amplification; increases the number of copies of proto-oncogenes proto-oncogene point mutation; protein product more active or more resistant to degradation
Tumor-suppressor genes •changes in genes that prevent uncontrolled cell growth (cancer growth stimulated by the absence of suppression) ex. p53 gene