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!

Definition of gene: A gene is the same as a segment of DNA that encodes a protein. You may also see the word cistron used. It is in practice the same as gene. The existence of a complex process which transfers the information from DNA to RNA (transcription) and thereafter to protein (translation) is the Central dogma. The protein formed is sometimes also called a polypeptide chain. There are several types of RNA. Those that encode proteins are called messenger RNA (mRNA).

11 The Central Dogma Cells store the genetic information to function and replicate in their DNA. When a protein is needed, DNA is transcribed into RNA, which in turn, is translated into a protein. FIGURE 2.1 Biotechnology by Clark and Pazdernik Copyright © 2012 by Academic Press. All rghts reserved.Biotechnology by Clark and Pazdernik Copyright © 2012 by Academic Press. All rghts reserved. There also exist enzymes (revers transcriptases) that can convert RNA into DNA, but none that can convert protein information back into RNA or DNA. Remember also that DNA can copy itself via assistance from certain proteins, particularly DNA polymerases.

12 The Structure of a Typical Gene Genes are regions of DNA that are transcribed to give RNA. In most cases, the RNA is translated into protein, but some RNA is not. The gene has a promoter region plus transcriptional start and stop points that flank the actual message. After transcription, the RNA has a 5’ untranslated region (5’ UTR) and 3’ untranslated region (3’ UTR), which are not translated; only the ORF is translated into protein. FIGURE 2.2 Biotechnology by Clark and Pazdernik Copyright © 2012 by Academic Press. All rights reserved. Overview of transcript and protein formation. ORF means open reading frame and is a very important concept. An ORF directly encodes a protein, but In eukaryotes most ORFs are discontinuous (split into parts called exons). UTR means untranslated part of RNA.

Illustration of what happens at the molecular level during RNA synthesis. Note that it is made in the 5’ to 3’ direction and that it is complementary to DNA

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

18 RNA Polymerase Synthesizes RNA at the Transcription Bubble RNA polymerase is a complex enzyme with two grooves. The first groove holds a single strand of DNA, and the second groove holds the growing RNA. RNA polymerase travels down the DNA, adding ribonucleotides that complement each of the bases on the DNA template strand. FIGURE 2.3 Biotechnology by Clark and Pazdernik Copyright © 2012 by Academic Press. All rights reserved.

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.

25 Eukaryotic Transcription Many different general transcription factors help RNA polymerase II find the TATA and initiator box region of a eukaryotic promoter. FIGURE 2.5 Biotechnology by Clark and Pazdernik Copyright © 2012 by Academic Press. All rights reserved. Transcription in eukaryotes involves many more proteins than in prokaryotes.

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