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