Gene Regulation and Structure

Grade 10 BiologySpring 2011

Describe how the lac operon is turned on and off

Summarize the role of transcription factors in regulating eukaryotic gene expression

Describe how eukaryotic genes are organized

Evaluate three ways that point mutations can alter genetic material

Prokaryotes have about 2,000 genes Humans have about 30,000 genes Not all of genes are transcribed and

translated at the same time So as not to waste energy and materials

Both are able to regulate gene expression depending on cell’s needs

E. ColiProkaryoteGene regulation well undestood- lac operon

gene

E. Coli is in the intestinal tract Lactose from milk enters and becomes

available to E. Coli E. Coli (bacteria) can absorb lactose and

break it down for energy Recognizing, consuming, and

breaking down lactose, into glucose and galactose, requires 3 different enzymes on 3 different genes

3 lactose metabolizing genes are located next to each other

Controlled by the same promoter site On-off switch

“turns on” (transcribes and then translates) the 3 genes when lactose is present

“turns off” genes when lactose is not available

Operator: Piece of DNA that overlaps promoter site

and serves as on-off switchAble to control RNA polymerase’s access to

3 lactose metabolizing genes Promoter:

Area in which RNA polymerase binds and allows the genes to be transcribed

Repressor protein:Protein that binds to an operator and

physically blocks RNA polymerase from binding to promoter site

Stops transcription of genes in operon

Operon:A group of genes that code for the enzymes

involved in the same function, their promoter site, and the operator that controls them all

Lac Operon:Operon that controls the metabolism of

lactose

Repressor protein turns the operon off Repressor protein binds to the operator

and blocks RNA polymerase from binding to the promoter site

Blocking of RNA polymerase stops the transcription of genes in the operon

Lactose binds to repressor protein and changes repressor proteins shape

Change of shape causes repressor protein to fall off of the operator

Now the RNA polymerase is free to bind to the promoter (no longer blocked)

RNA polymerase can transcribe the genes that code for the lactose metabolizing enzymes

By producing the enzymes only when the nutrient is available, the bacterium (E. coli) saves energy

Lets watch a video to review the lac operon!

Contain more DNA than prokaryotes Must continually turn genes on and off Operons are not common in eukaryotes Instead, genes with related functions

are often scattered on different chromosomes

Because there is a nuclear envelope that physically separates transcription from translation more opportunities for gene regulation

Gene regulation can occur:Before transcriptionDuring transcriptionAfter transcriptionAnd after mRNA leaves the nucleus or after

translation, when protein is functional

Most gene regulation in eukaryotes controls the onset of transcription When RNA polymerase binds to a gene

Use regulatory proteins- called transcription factors

But many more proteins involved and more complex

Transcription factors:Help arrange RNA polymerases in the

correct position on the promoter Gene can be influenced by many different

transcription factors

Enhancer: sequence of DNA that can be bound by a transcription factor Located thousands of nucleotide bases away

from promoter Loop in DNA may bring enhancer and its

attached transcription factor (activator) into contact with the transcription factors and RNA polymerase at the promoter

In eukaryotes many genes are interrupted by introns

Introns: long segments of nucleotides that have no coding information

Exons: portions of a gene that are translated (expressed) into proteins

After gene is transcribed, introns in mRNA are cut out by splicosomes

Splicosomes: complex assemblies of RNA and protein

Exons that remain are “stitched” back together by slicosome to form a smaller mRNA molecule

mRNA is then translated

Each exon encodes part of protein By having introns and exons cells can

occasionally shuffle exons to make new genes

Play an evolutionary role Thousands of proteins that occur seem

to have arisen from a few thousand exons

Some genes exist in multiple copies

Mutation: change in the DNA of a geneRare

Mutations in gametes can be passed on to offspring, those in body cells (somatic cells) cannot

Gene Rearrangements: mutations that move an entire gene to a new location Disrupt genes function,

gene is exposed to new regulatory conditions

Ex. You move to France and can’t speak French

Two types of Gene Rearrangements:1. Tranposition: genes are carried by

moving transposons 2. Chromosomal Rearrangement:

portions of the chromosome containing a gene may be rearranged during meiosis

Gene Alterations: mutations that change a gene Usually result in the placement of the wrong

amino acid during protein assembly Usually disrupts protein’s function

Three types of Gene Alterations:1. Point Mutations: single nucleotide

changes 2. Insertion Mutation: sizable length of

DNA is inserted into a gene Often result when mobile segments of DNA

(transposons) move randomly from one position to another on a chromosome

3. Deletion Mutation: segments of gene are lost

Often during meiosis

Point Mutation

Insertion Mutation

Deletion Mutation

Genetic message is read as a series of triplet nucleotide

Insertions and deletions can upset the triplet groupings

Ex. Delete the C from this sentence, keep letters in tripletsTHE CAT ATETHE ATA TE meaningless

Frameshift Mutation: mutation that causes gene to be read in the wrong 3-nucleotide sequence

http://www.youtube.com/watch?v=gqvYOr78THo

Activity Modeling Introns and Exons (p. 218) Procedure:

Place a 15-20cm strip of masking tape on your desk. Tape represents a gene.

Use 2 colours to write the words APPROPRIATLY JOINED on the tape exactly as shown. Space the letters so that they take up the entire length of the tape. The segments in one colour represent introns; those in the other colour represent exons.

Lift the tape. Working from left to right, cut apart the groups of letters written in the same colour. Stick the pieces of tape to your desk, making two strips according to colour and joining the pieces in their original order.

Activity Modeling Introns and Exons (p. 218)

Analysis Determine from the resulting two strips

which strip is made of introns and which is made of exons

Predict what might happen to a protein if an intron were not removed