COURSE DESCRIPTION

The structure of DNA was proposed by Watson and Crick in 1953. With an understanding of DNA structure, experimental evidence supported the proposal that DNA replicates in a semi-conservative manner. The central dogma states that genetic information is encoded in the DNA and transferred to the mRNA during translation. In addition to mRNA transcription, tRNA and rRNA are transcribed; tRNA is needed during translation while rRNA is a component of ribosomes. In eukaryotic transcription, pre-mRNA is synthesised and then processed to produce mature mRNA. Subsequently, through translation, the information on the mRNA is used to synthesise polypeptides, which are folded into functional proteins.

The nuclear genomes of eukaryotes differ greatly in size, number of genes and gene density from one another. The number of chromosomes differs between species and, in addition, certain organelles in eukaryotes possess small amounts of their own DNA. Eukaryotic genomes generally have a higher proportion of non-coding DNA to coding DNA. In addition to a large, circular chromosome, bacteria also have several plasmids. Even though bacteria reproduce asexually, they exhibit a great deal of genetic diversity through mutation and genetic transfer. In contrast to eukaryotic and prokaryotic genomes, the viral genome varies according to the types of virus: the genome may be DNA or RNA in nature and single-or double-stranded. For RNA viruses, they may possess either positive-sense RNA (i.e. identical to viral mRNA and thus can be immediately translated) or negative-sense RNA (i.e. complementary to viral mRNA and thus must be converted to positive-sense RNA by RNA polymerase before translation).

In prokaryotes, operons, like the trp and lac operons, regulate gene expression using repressible and inducible systems. Regulatory genes encode proteins that control transcription of structural genes. In eukaryotes, regulation of gene expression can occur at the chromatin level, transcriptional level, post-transcriptional level, translational level and/or even post-translation level. Basic molecular techniques allow scientists to study gene expression.

Learning Outcomes

Candidates should be able to:

1(a) Outline the cell theory with the understanding that cells are the smallest unit of life, all cells come from pre-existing cells, and living organisms are composed of cells

1(d) Describe the structure of a typical bacterial cell (small and unicellular, peptidoglycan well wall, circular DNA, 70S ribosomes and lack of membrane-bound organelles)

2(c) Describe how the information on DNA is used to synthesise polypeptides in prokaryotes and eukaryotes (description of the processes of transcription, formation of mRNA from pre-mRNA and translation is required)

2(d) Describe the structure and organisation of viral, prokaryotic and eukaryotic genomes (including DNA/RNA, single-/double-stranded, number of nucleotides, packing of DNA, linearity / circularity and presence/absence of introns)

2(g) Outline the mechanism of asexual reproduction by binary fission in a typical prokaryote and describe how transformation, transduction and conjugation (including the role of F plasmids but not Hfr) give rise to variation in prokaryotic genomes

2(i) Explain how gene expression in prokaryotes can be regulated, through the concept of simple operons (including lac and trp operons), including the role of regulatory genes, and distinguish between inducible and repressible systems (knowledge of attenuation of the trp operon is not required)

Use the knowledge gained in this section in new situations or to solve related problems.

LECTURE OUTLINE                                                                                                                                    

1. Bacterial Morphology and Cell Ultrastructure

1.1 Morphology                                                                                                                                

1.2 Cell Ultrastructure                                                                                                                      

1.2.1 Cell Surface Structures                                                                                                           

1.2.2 Cytoplasmic Structures                                                                                                        

1.2.3 Comparison of Prokaryotic and Eukaryotic Cell Ultrastructure                                     

1.2.4 Endosymbiont Theory                                                                                                           

2. Prokaryotic Genome

2.1 Bacterial Chromosome                                                                                                              

2.2 Plasmids                                                                                                                                       

3. Role of Genetic Material in Bacteria

3.1 Transmission of Genetic Material from Parent to Offspring – Binary Fission                   

3.2 Transfer of Genetic Material from One Cell to Another                                                        

3.2.1 Transformation                                                                                                                          

3.2.2 Transduction                                                                                                                              

3.2.2.1 Generalized Transduction                                                                                                     

3.2.2.2 Specialized Transduction                                                                                                       

3.2.2.3 Comparison of Generalized and Specialized Transduction                                              

3.2.3 Conjugation                                                                                                                                 

Concept Check 1                                                                                                                                                 

3.3 Prokaryotic Gene Expression                                                                                                       

3.3.1 Structure & Organisation of Prokaryotic Genes                                                                    

3.3.1.1 Grouping of Functionally Related Structural Genes – Operon                                         

3.3.2 Prokaryotic Transcription                                                                                                           

3.3.2.1 Stage 1: Initiation                                                                                                                     

3.3.2.2 Stage 2: Elongation                                                                                                                  

3.3.2.3 Stage 3: Termination                                                                                                                

3.3.3 Prokaryotic Translation                                                                                                               

3.3.3.1 Stage 1: Initiation                                                                                                                      

3.3.3.2 Stage 2: Elongation                                                                                                                   

3.3.3.3 Stage 3: Termination                                                                                                                 

3.4 Control of Prokaryotic Gene Expression                                                                                       

3.4.1 Coordinated Control of Gene Expression – Operon                                                                

3.4.1.1 Structure of an Operon                                                                                                             

3.4.1.2 Regulation of an Operon                                                                                                           

3.4.2 Example of a Repressible Operon – trp Operon                                                                        

3.4.3 Example of an Inducible Operon – lac Operon                                                                          

3.4.3.1 Negative Regulation of lac Operon – lac Repressor                                                               

3.4.3.2 Positive Regulation of lac Operon – Catabolite Activator Protein                                        

3.4.4 Comparison of Inducible and Repressible Operons                                                                    

Concept Check 2