Chromosomes, Gene Expression and Epigenetics

Why aren’t all cells the same if they have the same DNA?

All cell types have the same DNA in their nucleus, but not all DNA is “expressed” in all cells.

******************Let’s back up and learn a little more about chromosomes and gene expression.******************

Gene• A sequence of DNA that codes for a protein –

and thus determines a trait.• There are many genes on each chromosome.

Chromosomes• Thread-like structure made of DNA,

containing genetic information passed from generation to generation

• Located in the nucleus• Humans have 46 chromosomes – 23 from

mom, 23 from dad

Chromosomes are made of tightly – wound up DNA.

HISTONES are proteins around which DNA is coiled up.

When DNA is coiled up (condensed), it is called CHROMATIN.

Chromosome Structure• Each chromosome has a “P” arm (short arm)

and a “Q” arm (long arm)• The arms are attached with a CENTROMERE.• Each side of a chromosome is a CHROMATID.

Chromatid

Q Arm

P Arm

Centromere

• When DNA replicates (copies itself), there are two chromatids (SISTER CHROMATIDS) on each chromosome - held together with the centromere.

Ok, so what’s the deal with cells being different if they have the same DNA?

GENE EXPRESSION.• DNA is “expressed” when it is turned into

proteins - which MAKE us.

• Not all DNA is “expressed” (transcribed and translated into proteins) in every cell.

• Repressor proteins can stop transcription.

Gene RegulationOPERON: A group of genes that operate together.Parts of an operon:• PROMOTER: DNA sequences that are signals to “start”

transcribing RNA. A common one is TATA.• OPERATOR: DNA sequence where a REPRESSOR

PROTEIN can attach and stop (repress) RNA transcription.

Example: The lac Operon

1. E. Coli bacteria feed on lactose. In order to eat it, they must break it down into glucose and galactose.

2. E. Coli must produce the enzyme lactase (a protein) for this.

3. If lactose is present around the bacteria, it binds with a repressor protein and stops it from blocking transcription. Thus, the gene is transcribed into RNA, and is made into lactase.

http://www.google.com/url?sa=i&rct=j&q=lac+operon&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.scistudytime.com/2011/06/operon-model-lac-operon.html&ei=H1PRVPChO8iTyAS8roHACw&psig=AFQjCNFNcIV7zaguv2LjNfvuKpHoXvLjDQ&ust=1423090820070156

How do cells know what DNA to “express”?

• Cells listen for signals.

• Each signal shuts down some genes and activates others as it nudges a cell toward its final fate.

• Signals come from inside the cell, from neighboring cells, or from the outside world (environment).

Cell Signals for Expression

1. Once a signal reaches a cell, proteins carry information inside to the DNA like runners in a relay race.

2. The information is passed to a gene regulatory protein that attaches to a specific DNA sequence.

4. A regulatory protein attaches to specific DNA . It acts like a switch, activating genes or shutting them down.

5. Regulatory proteins use enzymes that add or remove epigenetic tags to the DNA, the histones, or both.

6. Epigenetic tags like methyl groups give the cell a way to "remember" long-term what its genes should be doing.

EpigeneticsEpi = Above, Outside

Epigenetics = Genetics controlled “outside” of the DNA sequence

• Epigenetic tags help or stop DNA from unwinding around histones.

• RNA can not be transcribed if DNA does not “unwind” from histones and “unzip”.

Check it out!

The Epigenome at a Glance (video): http://learn.genetics.utah.edu/content/epigenetics/intro/

Gene Control Simulation:http://learn.genetics.utah.edu/content/epigenetics/control/

The Epigenetics of Identical Twins (video)http://learn.genetics.utah.edu/content/epigenetics/twins/

Its in your jeans.