Q: Explain how protein synthesis can be controlled at a transcriptional level in prokaryotes

In prokaryotes, genes that encode enzymes of a metabolic pathway are usually clustered together on the chromosome in a region called an operon. A typical operon consists of regulatory sequences, like promoter, operator, terminator, and structural genes. Lac operon is an inducible operon that can be under negative control by lac repressor or positive control by CAP. In the absence of lactose, repressor binds to operator and blocks RNA polymerase from binding to promoter, thus no / low level of transcription of structural genes. In the presence of lactose, lactose will bind to repressor and change its conformation and prevent repressor from binding to operator, thus RNA polymerase can bind to promoter and transcription occurs. When both glucose and lactose are present, bacteria can selectively catabolise glucose instead of lactose. In the absence of glucose, the concentration of cAMP is high and cAMP binds to CAP and activates CAP, thus CAP binds to CAP-binding site and enhances rate of transcription. In the presence of glucose, concentration of cAMP is low and CAP not activated, thus CAP does not bind to CAP-binding site and transcription of structural genes occur as a low / basal rate.

Q: compare and contrast the prokaryotic and eukaryotic genomes

The eukaryotic genome is diploid but the prokaryotic genome is haploid. The eukaryotic genome has multiple origins of replication but the prokaryotic genome has only one origin of replication. The eukaryotic genome may have more than one chromosome but the prokaryotic genome has only one chromosome. The chromosomes in eukaryotic cells are linear but the chromosome in prokaryotic cells is circular. The eukaryotic genome size is much larger (10 to 100000 million bases) but the prokaryotic genome size is much smaller (0.6 to 10 Mb). The eukaryotic gene length is around one an a half times as long as the prokaryotic gene length. The eukaryotic genome is much more complex, with distinct regions such as the centromere and the telomere, whereas the prokaryotic genome is much simpler with no distinct regions. In the eukaryotic genome, the DNA is bound to histone proteins to form nucleosomes, which wind around each other to form chromosomes, but in the prokaryotic genome, DNA is bound to nucleoid associated proteins to form a DNA-protein complex. The eukaryotic genome is enclosed within a true membrane bound nucleus but the prokaryotic genome does not have a true nucleus, only a region called the nucleoid.

Q: Outline the differences between prokaryotic control of gene expression with the eukaryotic model

In the prokaryotic model, the regulation of transcription is by the control of operons. However, in the eukaryotic model, it is via the control of transcription factors. In prokaryotes, transcription can occur simultaneously with translation as there is no compartmentalisation by a nucleus. In eukaryotes, after transcription, RNA processing has to occur before translation of the mRNA. Transcription is carried out by 1 type of RNA polymerase in prokaryotes, but it is carried out by 3 different RNA polymerase enzymes in eukaryotes. Enhancers are absent in prokaryotes as promote regions on the DNA are regulated by operators, but in the eukaryotic model, enhancers are present to regulate the initiation of transcription from a promoter.

Control Elements

Control elements are non-coding DNA regions that regular transcription by RNA polymerase II. They come in two forms: enhancers and silencers.
Enhancers are a short region of DNA that can bind to proteins called activators. When such binding happens, transcription of a gene will be initiated. The gene can be some distance away from the enhancer, or even on a different chromosome. The increase in transcription is due to activators recruiting transcription factors, which enhances the binding of RNA polymerase.
A silencer is a DNA sequence capable of binding transcription regulation factors termed repressors. Upon binding, RNA polymerase is prevented from initiating transcription thus decreasing or fully suppressing RNA synthesis.

Q: Explain why mutations for antibiotic resistance spread so rapidly among bacteria

Frequent use of antibiotic kills many bacteria, creating a selection pressure. As a result, bacteria with mutations that give them resistance to a particular antibiotic have an advantage over the other bacteria that do not. They survive to reproduce more than the other types, and pass on this advantageous allele in greater numbers. The frequency of this allele therefore increases in subsequent generations, leading to an increase in frequency of resistant types in subsequent generations. As bacteria have a haploid genome, this alleles will be expressed and there will be no masking of any recessive alleles. In addition, bacteria multiply rapidly as they reproduce asexually by forming clones.