Chapter 12 Flashcards
(46 cards)
Overview of Gene Expression
We can look at gene function at two levels:
- molecular function of the protein product
- organism’s trait conferred by the gene
Two levels are connected – the molecular function affects the structure and function of cells to determine the trait
Inborn errors of metabolism
1908, Archbold Garrod first proposed relationship between genes and the production of enzymes
Studied patients with metabolic defects, or “inborn errors of metabolism”
Alkaptonuria
Patient’s body accumulates abnormal levels of homogentisic acid (alkapton)
Recessive pattern of inheritance
Hypothesis: “Disease is due to missing enzyme”
The nature of the genetic material was completely unknown at the time
Beadle and Tatum
Early 1940s – Beadle and Tatum became aware of Garrod’s work
They worked on Neurospora crassa, common bread mold
Minimum requirements for growth
Carbon source (sugar), inorganic salts, and biotin
Neurospora can synthesize everything else it needs from those precursors
Beadle and Tatum cont.
Wildtype – normal Neurospora, can grow on minimal medium
Mutant strains – unable to grow unless supplemented with specific substances (vitamins or amino acids)
Each single mutation resulted in the requirement for a single type of vitamin
Hypothesis: “One gene, one enzyme”
Beadle and Tatum cont.
Multiple mutants required arginine to grow
Could they grow if supplemented with precursors instead?
Fell into three groups based on requirements
Conclusion – Supports one gene, one enzyme hypothesis
Modern understanding
A modification of the “one gene, one enzyme” hypothesis
Enzymes are only one category of cellular proteins – genes also encode other proteins
Also, some proteins are composed of several polypeptides that work together for one function
Example: Hemoglobin composed of 4 polypeptides
“One gene, one polypeptide”
The Central Dogma 1
Transcription
Produces a transcript (RNA copy) of a gene
This messenger RNA (mRNA) specifies the amino acid sequence of a polypeptide
Translation
Process of synthesizing specific polypeptide on a ribosome using the mRNA template
DNA -> RNA -> Protein
Eukaryotes also have an intervening step called RNA processing where pre-mRNA is processed into active mRNA
DNA -> pre-mRNA -> mRNA -> Protein
Some genes do not encode polypeptides
an RNA is the final functional product
Structural RNAs
Regulatory RNAs
How do genes determine traits?
Genes constitute the genetic material
The “blueprint” for organisms’ characteristics
Structural genes code for polypeptides
One or several polypeptides act as a protein to play some role in the cell
Activities of proteins determine the structure and function of cells
Cellular activity, taken together in an organism determines their traits or characteristics
Transcription
Gene - An organized unit of base sequences that enables a segment of DNA to be transcribed into RNA and ultimately results in the formation of a functional product
Other genes code for RNA itself as a product:
Transfer RNA (tRNA) - translates mRNA into amino acids
Ribosomal RNA (rRNA) - part of ribosomes
pre-mRNA
In eukaryotes, the mRNA transcript before any biochemical modifications are made to it.
mature mRNA
In eukaryotes, transcription produces a longer RNA, called pre-mRNA, which undergoes certain modifications before it exits the nucleus; mature mRNA is the final functional product.
terminator
A sequence of DNA within a gene that specifies the end of transcription.
Signals the end of transcription
transfer RNA (tRNA)
An RNA that carries amino acids and is used to translate mRNA into polypeptides.
ribosomal RNA (rRNA)
An RNA that forms part of ribosomes, which provide the site where translation occurs.
promoter
A sequence of DNA within a gene that controls when and where transcription begins.
Signals the begining of transcription
regulatory sequence
In the regulation of transcription, a DNA sequence that functions as a binding site for a regulatory protein, which influences the rate of transcription.
Transcribed region:
Part of this region contains the information that specifies an amino acid sequence
The Three Stages of Transcription
initiation
elongation
termination
Intiation
The promoter functions as a recognition site for sigma factor. RNA polymerase is bound to sigma factor, which causes it to bind to the promoter. Following binding, the DNA is unwound to form an open complex.
Elongation/synthesis of the RNA transcript:
Sigma factor is released, and RNA polymerase slides along the DNA in an open complex to synthesize RNA. RNA polymerase slides along the template strand in the 3’ to 5’direction, while it synthesizes RNA In the opposite, 5’ to 3’, direction.
Termination:
When RNA’polymerase reaches the terminator, it and the RNA transcript dissociate from the DNA.
