1. Regulation of gene expression
The Code of Life: Topic 4
Regulation of gene expression

2. What determines your phenotype?
Gene expression!
You have 23 pairs of chromosomes.
In each pair you get one from your mother and one from your father.
Each chromosome in a pair holds all the same genes as the other.
So what determines which gene is expressed when you develop?
ie how do you get your mother's eyes or your father's nose?

3. Selective gene expression
Regulation of gene expression starts with cell differentiation during development.
Selective gene expression
All your cells contain ALL of your genes, they are not split up.
Depending on the function of the cell, some genes will be expressed and some will be permanently "turned off".
Can you think of a gene that is needed in red blood cells, but not in the pancreas?
Neuron (nerve cell)
muscle cells
liver cells

4. DNA Coiling Affects Gene Expression
Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix
DNA is negatively charged (phosphate groups)
Histone proteins are positively charged
This makes the DNA wrap around groups (8-9) of histones
Each wrapped group is called a nucleosome
The string then coils due to further charged-region interactions

5. Levels of chromatin packing
Single chromosome is about 4 cm in length (1000s of times diameter of nucleus!)
Wraps around histones (1/2 the mass of euk. chromosomes)
Nucleosome - like a bead on a string (wrapped around 8 histones)
Nucleosomes wrapped into tight helical fiber
Coiled further into a supercoil
Loops and folds further during mitosis to form a chromosome

6. Chromatin modifications
Remember…
DNA is slightly negative
Histone proteins have positive regions
This results in coiling of the DNA, making it difficult for enzymes to reach some portions of the DNA for transcription and keeping other portions exposed.
Acetylation
This can be modified by two processes:
Methylation: addition of methyl functional groups to histone proteins.
Acetylation: addition of acetyl functional groups to histone proteins.
Acetyl groups are negatively charged.
They neutralize the histones they attach to, so DNA will not wrap as tightly and will uncoil.
This exposes regions of DNA to transcription enzymes.

7. Epigenetics
The study of how environmental factors can change gene expression without a change in DNA.
Examples:
Diet (during development and beyond)
Stress
Hormones
Chemicals
Inheritance of methylation patterns

8. Extra slides

9. Regulation of gene expression
Regulation occurs at many stages:
Chromatin coiling and modification
Transcription
Processing
Translation

10. Regulation of gene expression
Transcription

11. Alternative RNA Splicing
Different final mRNA from the same RNA transcript
One gene = more than one polypeptide??

12. Gene
DNA
Exon 1
Intron
Exon 2
Intron
Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Polypeptide