Biology 210 - GENETICS, Spring, 1998

Biology 210

GENETICS

6 April, 1998

Chapter 11b (pages 479-505)
An Introduction to the Control of
Gene Expression in Eukaryotes

1. Differences between prokaryotic and eukaryotic Gene Expression.

2. The "Central Dogma" revisited.

3. Transcription in Eukaryotes

4. Eukaryotic DNA-binding protein motifs

1. Differences between prokaryotic and eukaryotic Gene Expression.

List of the 7 differences between eukaryotes and prokayotes (outlined on page 480 in your text).

1. In eukaryotes, one mRNA = one protein.

(in bacteria, one mRNA can be polycistronic, or code for several proteins).

2. DNA in eukaryotes forms a stable, compacted complex with histones. In bacteria, the chromatin is not in a permanently condensed state.

3. Eukaryotic DNA contains large regions of repetitive DNA, whilst bacterial DNA rarely contains any "extra" DNA.

4. Much of eukaryotic DNA does not code for proteins (~98% is non-coding in humans); in bacteria often more than 95% of the genome codes for proteins.

5. Sometimes, eukaryotes can use controlled gene rearrangement for increasing the number of specific genes. This happens rarely in bacteria.

6. Eukaryotic genes are split into exons and introns; in bacteria, genes are almost never split.

7. In eukaryotes, mRNA is synthesized in the nucleus and then processed and exported to the cytoplasm; in bacteria, transcription and translation can take place simultaneously off the same piece of DNA.

2. The "Central Dogma" revisited.

Remember that in eukaryotic cells, transcription occurs in the nucleus, and the mRNA must be exported to the cytoplasm for translation.

Figure 13-36 from Griffiths et al., 1996

In eukaryotes, the mRNA is modified at both ends - the 5' end is "Capped" with a modified GTP, and to the end is added a long "tail" of poly(A) (usually greater than 150 A's are added). This figure is taken from "The Biology Place"; you can click on it for a link there.

Once again (this is a review) - in bacteria, transcription and translation take place at the same time...

Figure 13-48a from Griffiths et al., 1996

In Eukaryotes, the process is a bit more complicated....
Figure 13-48b from Griffiths et al., 1996

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3. Transcription in Eukaryotes

A. Eukaryotic RNAPs

In bacteria, all mRNA is made from the same RNA polymerase. However, in eukaryotes, there are THREE DIFFERENT RNA polymerases:

1. RNA Polymerase I synthesizes rRNA.

2. RNA Polymerase II synthesizes mRNA. The mRNA molecules are only for one gene (monocistronic), whereas many prokaryotic mRNAs are polycistronic (code for more than one gene).

3. RNA polymerase III synthesizes tRNAs and also snRNAs (small nuclear RNAs) and small cellular RNAs.

The Eukcaryotic polymerases are much more complex than E.coli RNAP. This figure (Figure 13-35 from Griffiths et al., 1996) compares the different subunits for eukaryotic RNA Polymerases with that of E.coli RNAP.

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B. Eukaryotic RNA processing

In eukaryotes, the RNA is processed at both ends before it is spliced.

At the 5' end, a cap is added consisting of a modified GTP (guanosine triphosphate). This occurs at the beginning of transcription. The 5' cap is used as a recognition signal for ribosomes to bind to the mRNA.

At the 3' end, a poly(A) tail of 150 or more adenine nucleotides is added. The tail plays a role in the stability of the mRNA.

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C. Eukaryotic Transcription Initiation

There are 4 different types of proteins which must form a complex for initiation of transcription in eukaryotes.
Figure 18-47

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D. mRNA splicing

hit "reload" to see the animation

Figure 13-39 from Griffiths et al., 1996

Figure 13-38a from Griffiths et al., 1996

Figure 13-38b from Griffiths et al., 1996

Figure 13-40 from Griffiths et al., 1996

Figure 13-41 from Griffiths et al., 1996

Eukaryotic mRNA splicing

hit "reload" to see the animation

4. Eukaryotic DNA-binding protein motifs

To know some of the different DNA-binding domains of transcription factors:
            a. Helix-turn-helix
            b. homeotic box
            c. zinc fingers
            d. leucine zipper
            e. helix-loop-helix

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