Transcript Controls - Warren`s Science Page

CHAPTER 15.1
When Controls Come Into Play
AP Biology
Fall 2010
NEW VOCABULARY
Promotors: are short stretches of base
sequences in DNA where regulatory proteins
gather and control transcription of specific genes,
often in response to a hormonal signal
 Enhancers: binding sites where such proteins
increase transcription rates
 Methylation and Acetylation: are the names
for the addition of such groups to DNA or any
other molecule
 Cell Differentiation: nearly all of your body
cells became specialized in composition,
structure, and function

GENE EXPRESSION

Because all cells in your body have the same
genetic instructions……

Only a relatively small number of genes are active at
any given time in any given tissue
Which genes are expressed depends on the type
of cell, its responses to chemical signals, and
built-in control systems
 Regulatory proteins interact with DNA, RNA, or
actual gene products

SOME CONTROL MECHANISMS

In negative control systems, a repressor
protein binds to the DNA to block transcription
It can be removed by an inducer
 In this case a response slows or stops some activity

SOME CONTROL MECHANISMS

In positive control system, an activator
protein binds to the DNA and promotes initiation
of transcription

In this case a response enhances some activity
txn factor =
transcription
factor
VOCABULARY

Remember:
Promoters: short stretches of base sequences in
DNA where regulatory proteins control tanscription
of specific genes
 Enhancers: are the binding sites in DNA where
regulator proteins are found to be increasing
transcription rates
 Controls can be exerted by chemical modification –
methylation and acetylation – of DNA

Methyl groups (-CH3) are “painted” on parts of newly
replicated DNA to block access to genes
 Acetyl groups (-CH3CO-) are attached to DNA to make
genes accessible

POINTS OF CONTROL

There are four ways that genes in multicelled
organisms are turned off and on to maintain
homeostasis
POINTS OF CONTROL
1.
Controls before transcription:
Histone interactions that limit access of RNA
polymerase to genes
 Methylation and Acetylation of DNA functional
groups



Make histones loosen their grip
Polytene chromosome formation

Contain hundreds or thousands of side-by-side copies of
genes

Duplication and rearrangement of DNA sequences

Remember: access to a gene is under control!
POINTS OF CONTROL
2.
Control of transcript processing:
Removal of part of the RNA transcript (introns)
 Modification of RNA (attachment of proteins)

So it can pass through the nuclear pores
 Controls when the mRNA transcript reaches a ribosome



Special codes in the mRNA that determine where in
the cell’s cytoplasm it will be used
Y-box protein that can bind to a transcript to block its
translation
When phosphorylated Y-box proteins bind and help
stabilize mRNA
 When many of the proteins bind to a transcript, they block
its translation
 Control mechanism in mRNA inactivation

INTRONS AND EXONS
(PG. 232 MUSCLE CELL EXAMPLE)
POINTS OF CONTROL
3.
Controls of translation:

Interactions of ribosomal subunits and initiation
factors and the stability of mRNA transcripts before
degradation begins
POINTS OF CONTROL
4.
Controls after translation:

Addition of phosphate to the Y-box proteins

Activates Y-box
Other controls activate, inhibit, and stabilize diverse
molecules that take part in protein synthesis
 Allosteric control of enzymes in metabolic pathways

REMEMBER ALLOSTERIC?
SAME GENES, DIFFERENT CELL LINEAGES

All body cells have the same genes, but the cells
of different tissues are differentiated (specialized)
because of selective gene expression
Every body cell arose by mitotic division from the
same fertilized eggs
 Nearly all of your body cells become specialized in
composition, structure, and function (Cell
Differentiation)


For example:


Red blood cells transcribe genes for hemoglobin
Genes in the eye make crystallin for the transparent
lens
FIGURE 15.3 PAGE 233
SUMMARIZES MAIN CONTROL POINTS OVER
GENE EXPRESSION