Upload
beverly-cummings
View
216
Download
0
Embed Size (px)
Citation preview
CHAPTER 15.1 When Controls Come Into Play
AP BiologyFall 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 233SUMMARIZES MAIN CONTROL POINTS OVER GENE EXPRESSION