Central Dogma of Genetics
I. Central Dogma of Genetics
Refer to Figure 7.1 on page 176.
II. Gene
A. Definition of gene
A segment of DNA that codes for a specific product.
The final product for most genes is a protein.
Some genes produce rRNA (ribosomal RNA) or tRNA (transfer RNA) as a final product.
See Figure 7.3 on page 178.
B. Anatomy of a gene
The gene is only found on one of the DNA strands.
The gene has 3 parts: a. Promoter: Beginning of the gene. It is a start signal for transcription. b. Coding Region: The part of the gene that contains the code to build the product. This is where the transcription occurs, as it is the part that gets copied. c. Terminator: End of the gene. It is a stop signal for transcription.
In the provided drawing:
Green represents the promoter,
Blue represents the coding region,
Red represents the terminator.
III. Transcription
A. Definition
Transcription is the production of a complementary RNA copy from a DNA template.
B. Enzyme
Transcription is carried out by RNA polymerase.
C. Steps
Refer to Figure 7.10 on page 183.
Initiation
a. RNA polymerase binds to the promoter region of the gene.
b. A small bubble is opened up within the DNA.Elongation (named because we are making a strand of RNA, which gets longer as we add nucleotides one at a time) a. RNA polymerase moves along the DNA and makes a complementary copy of RNA from one strand (template strand).
DNA template: TCCAGCCCATTTAAGC
RNA copy: AGGUCGGGUAAAUUCG
b. Nucleotides are added onto the 3’ end of the growing RNA chain, so RNA synthesis occurs from 5’ to 3’ direction.
Termination
a. When RNA polymerase reaches the terminator, transcription stops.
D. 3 types of RNA produced by transcription
messenger RNA (mRNA)
ribosomal RNA (rRNA)
transfer RNA (tRNA)
IV. Translation
A. Definition
Translation is the process of converting the mRNA sequence into a corresponding amino acid sequence.
B. Requirements for translation
mRNA
a. Contains codons.
b. 3 nucleotides = 1 amino acid.Refer to Table 7.3 on page 184.
The code is degenerate (redundant), meaning that there is more than one codon for most amino acids.
Key codons include the start codon (AUG), which provides the start signal for translation and codes for methionine, and 3 stop codons that signal termination of translation.
Practice assignment:
mRNA: AUG CCG GCU GAU GAC GGG UAA
Amino acids: .
tRNA (refer also to Figure 7.13 on page 185)
a. The tRNA has a specific amino acid attached to it.
b. It also has an anticodon (labeled attachment site in the above picture). This consists of 3 tRNA nucleotides that are complementary to a specific codon. This allows the cell to match the correct amino acid with the appropriate codon.Hydrogen bonds form between the codon and the anticodon.
Important note: When looking up amino acids in Table 7.3 on page 184, always look up the codon, not the anticodon.
You will be given both mRNA & tRNA sequences during the test and asked for the correct amino acid sequence.
Ribosome (composed of rRNA & proteins)
a. The ribosome has 2 subunits: the small subunit (30S) and the large subunit (50S).
b. The ribosome has 3 sites for tRNA molecules: A site, P site, and E site.Refer to Figure 7.15 on page 186.
C. Steps of translation
Initiation
Refer to Figure 7.15 on page 186.
a. The ribosome assembles at the start codon (AUG).
b. The 1st tRNA (initiator-tRNA) brings methionine to the P site.
Elongation
Refer to Figure 7.16 on page 187. a. Codon recognition: A tRNA brings its amino acid to the A site. b. A peptide bond forms between the amino acids. c. Translocation: The ribosome shifts position by moving one codon forward.
The tRNA that was in the P site moves to the E site and exits the ribosome.
The tRNA that was in the A site moves to the P site, leaving the A site open for another amino acid.
This cycle of elongation repeats hundreds of times.
Termination
Refer to Figure 7.17 on page 187.
a. When a stop codon enters the A site, translation terminates.Note: Understanding the steps of translation is crucial as certain antibiotics can prevent codon recognition, peptide bond formation, and translocation.
Watch the video: Translation on YouTube. (Ignore mentions about mRNA cap and tail, as they pertain only to eukaryotes.)
V. Transcription & Translation in Prokaryotes vs Eukaryotes
A. In prokaryotes, transcription & translation occur simultaneously.
This occurs because prokaryotes do not possess a nucleus.
In eukaryotes, transcription happens in the nucleus, whereas translation occurs in the cytoplasm where ribosomes are located. Thus, these processes are separate.
In prokaryotes, both transcription & translation occur in the cytoplasm, allowing them to occur at the same time.
B. Polyribosomes
The simultaneous action of transcription and translation, along with polyribosomes (where many ribosomes can associate with each mRNA transcript), enhances the speed of gene expression.
Refer to Figure 7.18 on page 188.
Note the ribosomes attaching to mRNA transcripts before they are completely formed.
VI. Regulation of Transcription
A. Bacterial Gene Regulation
Bacteria only express those genes that are necessary for maximum growth under appropriate environmental conditions.
Gene expression pertains to the processes where the gene has been transcribed and translated.
(Discussion on gene expression will continue in the virus chapter.)
B. Gene Expression Control
Organisms have the ability to control which genes are expressed at both transcription and translation stages.
Regulating transcription is more common compared to regulating translation.
Specific methods of transcription regulation include the inducible operon.
Refer to pages 193 and Figure 7.22 on page 194.
VII. Regulation of the Lactose (Lac) Operon
A. Gene Abbreviations
Genes have a 3-letter abbreviation in scientific nomenclature.
If the letters are all lower-case and italicized (or underlined in handwriting), this refers to the gene itself made of DNA.
If multiple genes share the same abbreviation, a capital letter is added to differentiate them.
Examples: lacA, lacB, lacC, lacD, etc.
If the first letter is capitalized and not italicized, it refers to the protein expressed from that gene.
Examples: LacA, LacB, LacC, LacD.
B. Operon
An operon is composed of multiple genes sharing a common promoter.
The genes within the operon usually perform a common function.
For example, the lac operon in E. coli consists of 3 genes (lacZ, lacY, & lacA) that help metabolize the sugar lactose.
The operon contains an operator region that serves as a switch to turn transcription of the genes on or off.
The operon allows for coordinated control of related genes, enhancing efficiency similar to a light switch controlling multiple light bulbs simultaneously.
C. The Lac Operon
The lac operon is found in E. coli and consists of the following genes:
lacZ, lacY, & lacA which are involved in lactose metabolism.
The operon has a promoter where RNA polymerase binds, and the operator region plays a role in regulating the transcription.
The operon contains 3 genes in its coding region.
D. Regulation by LacI Protein
Near the operon is a gene called lacI.
This gene produces a repressor protein that inhibits (represses) transcription of the operon.
E. Inducibility of the Lac Operon
The lac operon is normally turned off (not transcribed).
The cell avoids wasting energy on protein production required for lactose metabolism if lactose is absent.
Transcription of the lac operon can be induced in the presence of lactose.
F. Conditions Affecting Transcription
Condition: Lactose is absent
The repressor protein binds to the operator of the lac operon, preventing RNA polymerase from reaching the coding region.
Condition: Lactose is present
The cell converts some lactose into allolactose, a structural isomer of lactose.
Allolactose acts as the inducer, stimulating transcription.
Allolactose binds to the repressor protein, causing a conformational change that inactivates the repressor.
The inactive repressor cannot bind to the operator.
As a result, transcription occurs.
Recommended to watch the video posted in D2L about the lac operon.