Transcript Slide 1
The DNA in a bacterial virus – enormous condensation is needed for the virus head to accomodate all its DNA. In Escherichia coli the DNA is about 1 med mer long, while the cell is close to 1 μm. Here the DNA information also has to be read! In human cells the total length of the DNA is around 1 m! This requires very ”advanced” methods of packaging to both have enough space in the nucleus and at the same time allow reading of the information A chromosome in which packaging has been partly disrupted DNA in eukaryotes (but not in bacteria and Archae) is twisted around protein complexes called histones. They are positively charged proteins that interact with the negatively charged DNA. Each ”ball” is called a nucleosome. An analogy to DNA supercoiling Unwinding in one part leads to supercoiling in another Supercoiling can be either positive or negative The Central Dogma – this is probably the most important slide in the course! A gene is the same as a segment of DNA that after transcription and translation gives rise to a specific protein (polypeptide chain). You may also see the word cistron used. It is in practice the same as gene. Illustration of what happens at the molecular level during RNA synthesis The 3D structure of a bacterial RNA polymerase Initiation of transcription in E. coli. The process is much more complex in eukaryotes in that many accessory proteins are involved The transcription of DNA by RNA polymerase The completion of the transcription cycle The DNA elements required for transcription in prokaryotes. An UP element may or may not be present The transcript has the same sequence as the non-template strand except that T is substituted by U. There are many transcription factors in bacteria, and the numbers vary a lot between species. In eukaryotes there are an enormous number of such factors. Several genes can be transcribed as a unit in bacteria. This is not common in eukaryotes. Such a unit (cluster of cotranscribed genes) is called an operon. Transcription in eukaryotes is much more complex than in bacteria, partly because many more protein factors are involved Transcripts in eukaryotes are heavily modified after transcription, by capping, polyadenylation and splicing Eukaryotic genes are made up of exons and introns. Only the exon parts encodes the corresponding protein Splicing may occur in different ways, so that several different proteins are made from one specific mRNA. The varying processes may takwe place in different tissues of a body. Introns are spliced ourt by autocatalysis or protein-assisted catalysis. NB! This shows that RNA alone can be catalytic (acts like an enzyme). Genes can be located on different DNA strands