Central dogma of genetics

Lecture 4

The conversion of DNA to Proteins

Step 1: Transcription• The DNA is double stranded with a

5’ to 3’ sequence complemented by a 3’ to 5’ sequence. (get a diagram lecture 3):

• The 5’ to 3’ is the coding strand (sense strand)

• The 3’ to 5’ template strand. (antisense strand)

• Messanger RNA (mRNA) is exact copy of the coding ( sense strand ) apart from Thymine is replaced by Uracil (T replaced by U)

•

Adapted from

Transcription• In prokaryotic cells the mRNA is translated into amino acid peptides as shown in Fig 12.1

above.• In Eukaryotic cells the transcription produces (pre-mRNA or nuclear mRNA) [RNA in figure

below] and it needs further processing before it is translated. • The “genes” of Eukaryotic DNA can be divided into regions called exons and introns • The exons will be translated while the introns are “spliced” removed from the pre MRNA and

will not translated.• This splicing process is more complex than shown below as numerous splicing process can

occur leading to numerous mRNA strands from the same pre-mRNA strand: the process is referred to as alternative splicing.

Steps in transcription in Eukaryotes

Alternative splicingThe pre-mRNA can be spliced into a number of alternative spliced mRNA sequences (isoforms).

The pre-MRNA can produce many isoforms

It is the primary reason that ~25,000 genes produce ~2,000,000 proteins

Exons from pre-mRNA can be spliced in different ways to give structurally and functionally different mRNA and proteins [4].

40-60% of genes have alternative splicing forms.

Frequencies of splices can vary 1 - thousands.

Encoding proteins at nodes highly connected interaction networks e.g. neural tissue

Adapted from [3]

Types of Alternative splicing

A more comprehensive description A.S. can be found at ref [5]

Effects of alternative splicing [8]

Alternative splicing: the effects

• Alternative splicing can lead to:1.use of a different site for translation initiation (alternative

initiation),2.a different translation termination site due to a frameshift

(truncation or extension), or the addition or removal of a stop codon in the alternative coding sequence (alternative termination).

3.Alternative splicing can also change the internal region because of an in-frame insertion or deletion.

mRNA -> AA: translation

Adapted from ref [1] p

The genetic code

Initiation / terminationThe codon (triplet) code where the polypeptide begins is AUG

Two situations where this codon can occur: at the beginning or in the middle; if the AUG is at the beginning the methionine is chemically altered.

There are 3 stop codons; the tRNA associated with these have no AA attached and so terminate

A more detailed descriptin can be found at ref [1] p266/267

DNA replicationDNA replication involves the following process:

A enzyme (a molecule that increases the natural the rate of a chemical reaction) called DNA polymerase attaches to the DS DNA

It then unwinds the DNA and copies each strand.

Basically DNA replication is different for the two classes of cells prokaryotics/Eukaryotics : Why . Explain briefly how there are dealt with.

Note you must give a reference in answering this question [ wikipedia must not be the primary source]

To be completed in a lab.

References

• [2] http://www.di.uq.edu.au/sparqtransctrans accessed on the 30/09/2011

• [3] http://www.ncbi.nlm.nih.gov/Class/MLACourse/Modules/MolBioReview/alternative_splicing.html

• [4] Blencowe, B.J. 2006. Alternative Splicing: New Insights from Global Analyses. Cell 126 37-47.

• [5] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1370565/