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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