Gene Expression

Cell DifferentiationCell types are different

because genes are expressed differently in them.

Causes: Changes in chromatin structure Initiation of transcription RNA processing mRNA degradation Translation Protein processing and

degradation

Operons Unit of genetic function

consisting of related clusters of genes with related functions

A “switch” that controls enzyme production

Coded for by one transcription unit

Repressible Operons (The trp Operon)

Repressible Operon: always operates unless a repressor turns it off. promoter: RNA polymerase binding site;

begins transcription operator: controls access of RNA

polymerase to genes transcription stops here when repressor is in place

repressor: protein that binds to operator

and prevents attachment of RNA polymerase

Sometimes, a corepressor must be in place for the

repressor to be active Tryptophan (a.a.) synthesis

Transcription is repressed when tryptophan binds to the repressor, which connects to the operator

Inducible Operons (The lac operon)

Inducible Operon: Always off unless an inducer is present Inducer attaches to the repressor

and causes it to move so that transcription can occur

Lactose metabolism (lac operon) lactose not present: repressor

active, operon off; no transcription for lactose enzymes

lactose present: repressor inactive, operon on

inducer molecule inactivates protein repressor (allolactose)

Chromatin Complex of DNA and proteins DNA Packing

histone protein (+ charged amino acids ~ phosphates of DNA are - charged)

Nucleosome ”beads on a string” basic unit of DNA packing

Heterochromatin highly condensed interphase DNA

(can not be transcribed) Euchromatin

“true chromatin” less compacted interphase DNA

(can be transcribed)

Histone Modification

Genes within highly packed heterochromatin are usually not expressed

Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression Acetylation prevents histones

from packing tightly, which allows genes to be expressed.

Methylation causes histones to pack tightly so that genes are not expressed.

Epigenetic Inheritance Expression of traits is not necessarily

related to the nucleotide sequence Some individuals may express traits

from their genes where others will not based on histone modifications One twin may express a trait or get a

disease that the other does not, despite same genes Schizophrenia Some cancers Etc.

Regulation of Transcription

Control Elements- noncoding DNA that regulate binding proteins

Enhancers- segments that influence how a gene is expressed Often placed far from the

actual gene

RNA and Protein Processing

Alternative RNA splicing Different mRNA molecules

formed from the same primary transcript

mRNA degradation Protein processing Protein degradation

proteasomes

Cell Differentiation

How cells become specialized in structure and function.

Determinants exist in the egg cell Influence the expression of

characteristics in different regions of cells

Once cells divide by mitosis, specific regions of the embryo will express genes differently

(b) Induction by nearby cells(a) Cytoplasmic determinants in the egg

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two-celledembryo

Signalmolecule(inducer)

Signaltransductionpathway

Early embryo(32 cells)

Nucleus

NUCLEUS

Signalreceptor

Body Plan Setup

Pattern Formation

cytoplasmic determinants

inductive signals determine spatial

organization of tissues

Biology of Cancer Oncogene- cancer-causing genes Proto-oncogene- normal cellular genes How does a proto-oncogene become an oncogene?

movement of DNA; chromosome fragments that have rejoined incorrectly amplification; increases the number of copies of proto-oncogenes point mutation; protein product more active or more resistant to degradation

Tumor-suppressor genes changes in genes that prevent uncontrolled cell growth (cancer growth stimulated

by the absence of suppression)

ras and p53 ras

Produces Ras proteins Hyperactive Ras protein

causes cell cycle to continue (increased cell division)

Mutations involved in 30% of all cancers

p53 Tumor-suppressor gene Activated by DNA

damage Turns on DNA repair or

activates “suicide” genes Mutations involved in

50% of all cancers