Sunday, 6 December 2009

Biochemistry Exams - 2008/2009 3rd year Undenominated Science Semester I - Question 2

Hello Again,

After this short invitation/background text:) Question 2 from 3rd year Undenominated science exam paper will be exposed (info about the paper can be found here 2008/2009 3rd year Undenominated Science Semester I).

Ok here we go !!

Question 2 Discuss the role of histone proteins in packaging the genome and regulating its function.

Short background, histones are:
- small, alkaline proteins (positively charged beacause of high number of lysines and arginines in the sequence)
- there are five classes (H1/H5, H2A, H2B, H3, H4)
- found in nucleous
- interact with other histones and DNA

Histones interact to form an octamer structure which is called a nucleosome scfaffold. It is made of  H2A/H2B tetramers and another tetrameric H3/H4 subunit. Because histones are positively charged (a lot of Lys and Arg in the sequence) the octamer easily interact (ionic interaction) with the DNA (which is negatively charged). DNA/octamer complex is called a nucleosome (see figure below).

Picture taken from Nucleosome Structure.

Nucleosomes allow DNA compaction necessary to fit genomes of eukaryotes inside cell nuclei.
Remember that each group (H1/H5, H2A, H2B, H3 and H4) contain many histone isoforms and all of them can form octamer structure. Depending on that structure DNA wrapped around it may have different properties.
Keep also in mind that histones like other proteins can be post-translationaly modified. There are different modifications detected at histones: methylation, acetylation, phosphorylation, ubiquitination, sumoylation, citrulination and ADP-ribosylation. Each modification and combinations of them have an impact on histone structure and function. Histone modifications affect:
- DNA replication,
- DNA compaction (euchromatin and heterochromatin)
- gene transcription,
- DNA repair - when DNA is damaged histones are modified to recruit DNA repair proteins,
- DNA/protein complexes - their assembly or disassembly,
- their stability - when histones have to be removed, their modification can target them for degradation and when they are needed they are targeted to DNA,
- and more.

Before you finish have a look at theis animation Biology Animations - Histone modification.

I hope you enjoy.

Soon next question:)


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