EUKARYOTIC GENE CONTROL

THE BIG QUESTIONS

• How are genes turned on and off? • How do cells with the same DNA/genes differentiate to perform completely different and specialized functions?

GENE EXPRESSION

• The control of gene expression takes place along a specific pathway.

•  1. Packing/unpacking DNA •  2. Transcription •  3. mRNA processing •  4. mRNA transport •  5. Translation •  6. Protein processing •  7. Protein degradation/mRNA degradation

1. DNA PACKING

• The double stranded DNA needs to coil and fold to fit inside of the nucleus.

• Double stranded DNA • DNA wraps around a histone protein

forming a nucleosome •  Looped domains • Chromosome

HISTONE/NUCLEOSOME

Positively charged amino acids on the histone proteins bind to the negatively charged DNA.

LOOPED DOMAIN

CHROMOSOME

DNA PACKING CONTROLS TRANSCRIPTION

• Heterochromatin (H)-The DNA is tightly packed and will appear dark in an image.

• No transcription can take place so the genes in that region are “turned off”

• An example would be the insulin gene in

brain or bone cells.

HETEROCHROMATIN

DNA METHYLATION

•  Methylation of DNA in the promoter region blocks transcription factors so the genes are “turned off”

•  Occurs when a methyl group (CH3) is attached to a cytosine (C)

•  This process in nearly permanent. •  ex. Inactivation of mammalian X chromosome=Barr body

DNA PACKING CONTROLS TRANSCRIPTION

• Euchromatin (E)-The DNA is loosely packed and will appear light in an image.

• Transcription can take place so the genes in that region are “turned on”

EUCHROMATIN

LOOSEN THE DNA BY HISTONE ACETYLATION

•  Attachment of an acetyl group (COOCH3) to a lysine of the histone which will help unwind the DNA.

•  The DNA will be loosely wrapped which will allow for transcription and the genes to be “turned on”.

2. TRANSCRIPTION INITIATION

• Control regions of the DNA •  Enhancer is a short region of DNA bound by

regulatory proteins to increase the likelihood of transcription.

•  Promoter is a nearby control sequence of DNA where transcription factors and RNA Polymerase can bind. This is called the transcription complex.

•  The DNA will fold forming the transcription complex and the mRNA can begin to be produced.

3. POST TRANSCRIPTIONAL CONTROL

• Removal of introns using spliceosomes.

• Add a 5’ G cap

and 3’ poly A tail.

4. REGULATION OF MRNA DEGREGATION

•  Life span of mRNA determines the amount of protein synthesis.

•  The mRNA can last from hours to weeks.

DEATH OF MRNA

• Small interfering RNA (siRNA)

•  Short segments of RNA that bind to mRNA and create double stranded mRNA.

•  Triggers the cell to degrade the mRNA. The “death tag” for mRNA.

• Causes gene silencing. The gene is turned off so no protein is produced.

ACTION OF SIRNA

siRNA

double-stranded miRNA + siRNA

mRNA degraded functionally turns gene off

Hot…Hot new topic in biology

mRNA for translation

breakdown enzyme (RISC)

dicer enzyme

5. CONTROL OF TRANSLATION

• Block initiation of translation tag •  Regulatory proteins attach to the 5’ end of mRNA •  Prevents the attachment of ribosomal subunits

and initiator tRNA •  Blocks translation of mRNA to protein

6. PROTEIN PROCESSING AND DEGRADATION

• Protein processing •  Folding, cleaving, adding sugar groups and

targeting for transport.

• Protein degradation •  Ubiquitin-Tags the protein for degradation by

proteases •  Proteasomes-Responsible for the degradation of

the protein.

UBIQUITIN

• “Death Tag” • A 76 amino acid polypeptide that

marks unwanted proteins with a label to be broken down by proteasomes.

PROTEASOMES

• Protein-degrading machine. • Breaks down any protein into 7-9

amino acid fragments that can be recycled.

THE WHOLE STORY

initiation of transcription

1

mRNA splicing

2

mRNA protection 3

initiation of translation

6

mRNA processing

5

1 & 2. transcription - DNA packing - transcription factors 3 & 4. post-transcription - mRNA processing - splicing - 5’ cap & poly-A tail - breakdown by siRNA

5. translation - block start of translation

6 & 7. post-translation - protein processing - protein degradation

7 protein processing & degradation

4

4

Gene Regulation