gene expression. cell differentiation cell types are different because genes are expressed...
TRANSCRIPT
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