Chapter 16 Biology (Final)

Gene Regulation

Process of controlling which genes in a cell’s DNA are expressed

Differential gene expression

Expression of different sets of genes by cells with the same genome

Differential gene expression allows different cells to

Carry out different functions

Trancsiption happens in the

Nucleus 

Translation happens in the

Cytoplasm

Prokaryotic DNA is organized into a circular chromosome located in

Nucleoid region of cytoplasm

Genes with similar function or in the same biochemical pathway are organized in

Block called operons

Operons consist of 

Promoter

Operator

Cluster of functionally related genes

Promoter

Nucleotide sequence within a gene that binds RNA polymerase to initiate transcription

Operator

Sequence of nucleotides near the start of an operon that acts as the key control “on-off” switch 

Cluster of functionally related genes that are

Trancribed in a single mRNA strand containing multiple start & stop codons

Operons are regulated by

Repressors and activators

Repressors

Proteins that bind to operator to suppress transcription

Negative regulation

Proteins bind to the operator turning off expression 

Activators

Protiens that bind to operator to activate transcription

Positive regulation

Positive regulation

Protein bind to operator turning on expression

Negative regulation by a repressor

The trp operon

trp operon controls genes needed for

Tryptophan biosynthesis

Tryptophan is scarce when

Repressor protein is inactive

Operon is on

Trptophan is plentiful when

Tryptophan molecules bind the repressor protein at the operator sequence

Operon is off

Tryptophan is a corepressor

Corepressor 

Binds to and activates the repressor

Negative regulation example

Lac operon

In the absence of lactose

A repressor binds the operator blocking transcription

In the presence of lactose 

Lactose binds to the repressor preventing it from binding the operator 

Lactose acts as an

Inducer

Inducer

Inactivates the repressor and turns on transcription

Positive rehulation by an activator

Lac operon 

In the absence of cyclic AMP

The catobolite activator protein does not bind which causes transcription to occur at a low rate 

When glucose levels increase

cAMP binds to CAP protein complex

cAMP/CAP binds to a region near lac promoter 

Increase transcription of genes needed to metabolize lactose 

Eukaryotes do not have

Operons

Eukaryotic genes have more 

complex regulation mechanisms

Gene regulation can occur at

Several different steps

Different steps of gene regulation

Chromatin modification

Transcription

mRNA processing

mRNA degradation

Translation

Protein processing

Protein Degradation

Chromatin modification deals with how

Chromosomes are packaged

Organization of chromatin

DNA is folded around histone proteins to create nucleosomes

In loosely packed euchromatin, nucleosomes control access of proteins to DNA 

Genes with densely packed heterochromatin are usually not expressed

Epigenetic Regulation

Control of gene activity through modification to proteins and DNA that do not alter the nucleotide sequence but does alter gene expression

Epigenetic regulation condenses chromatin into

Hetero chromatin which reduces transcription

Epigenetic inheritance

Chemical modifications are passed on to daugher cells

Chemical modification of histones and DNA

Epigenetic regulation and inheritance

Regulation of transcription is the most common form of 

Gene regulation in eukaryotes

Transcription factors

Proteins that help turn specific genes on or off by binding to DNA 

Proteins bind to

specific DNA sequences

When proteins bind to specific DNA sequences what happens

They regulate the activity of RNA polymerase

Activators

Enhance or turn on a genes transcription

Repressors

Turn off gene transcription

RNA polymerase II requires

Transcription factors to initiate transcription

Some transcription factors bind to the

TATA box in the promoter

Some transcription factors bind to proteins including what

Other transcription factors and RNA polymerase II

Enhancer

DNA sequence that promotes transcription

What is each enhancer made of

Short DNA sequences called distal control elements

Activators bind to distal control elements and interact with

Transcription factors and other proteins

DNA bending protein changes

Shape of DNA and properly orients enhancer region

Post-transcription regulation allows a cell to 

Rapidly fine-tune gene expression in response to environmental changes 

Alternative RNA splicing

Different mRNA molecules are produced from the same primary transcript

mRNA degradation fine-tunes

Protein levels without altering transcriptions

mRNA degradation regulates how

Long transcripts are available for translation

RNA-Bidning proteins at the 5’ and 3’ untranslated regions influence the RNA

Stability

Regulation of translation

Translation can be blocked by regulatory proteins 

Regulatory proteins bind to 

Sequences of mRNA

After tanslation

Polypeptides undergo processing

The length of time each protein functions is

Regulated by selective degradation

Phosphorylation is used to

Activate protein

Genes associated with cancer are

Mutations that alter normal cell growth and division

Proto-oncogenes

Gene that regulate cell growth, division, and differentiation

Oncogene

A proto-oncogene that has mutated and drives uncontrolled cell division

Mutation in the what are common in human cancer?

Ras proto-oncogene

Tumor-supressor genes 

Gene whose protein product inhibits cell division and prevents uncontrolled cell growth 

Tumor suppressor gene p53

Mutated in over 50% of all cancer types

Proto-oncogene ras codes for

Ras protein that acts as a molecular switch in cell signaling pathways

Ras protein

G protein that relays a signal from a growth factor receptor on the cell surface to stimulate cell division

Mutation in the ras gene can lead to

Production of a huperactive Ras protein 

The mutant oncogene produces a protein that 

Does not require a growth factor to be activated 

The p53 gene codes for transcription factor that

Sense DNA damage

When p53 is activated it

Halts the cell cycle to allow DNA repair

If damage is irrepairable

p53 triggers apoptosis

Multiple mutations are generally needed for

Full-fledged cancer

A cancerous cell is usually characterized by 

At least one active oncogene and the mutation of several tumor-suppressor genes 

Individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes, leads to

Predisposition for cancer

During embryonic development

A fertilized egg gives rise to many different cell types

Differential gene expression leads to 

Different cell types

Genes are regulated

Differently in each cell type

Cell differentiation

Process by which cells become specialized in structure and function

Cytoplasmic determinants are

Maternal substances in the egg that influence early development

An egg’s cytoplasm contains

RNA, proteins, and other substances that are unevenly distributed

As the zygote divides by mitosis

Cells contain different cytoplasmic determinants which leads to differential gene expression 

Environment around the cell contribute to

Cell differentiation