Nucleic Acids- Self Replication of DNA, RNA Synthesis and the Genetic Code

What is meant by semiconservative replication of DNA?

In semiconservative replication of DNA, each daughter molecule has one strand from the old DNA (hence "conservative") and one newly synthesised strand (hence the process is only "semiconservative").

What are the requirements for DNA replication?

DNA replication requires many different things. For starters, it requires origin of replication sequences on the DNA to "tell" other machinery in the cell where to begin replication. It also requires helicases, which open up the helix, as well as other proteins that bind onto the DNA and keep the DNA "unpeeled" while synthesis is taking place. A short primer sequence of RNA is required to bind to the DNA. DNA polymerase, an enzyme that synthesises new DNA strands as it moves down an existing strand, is also required, as are new nucleotides to be added. (DNA polymerase can only add nucleotides to an existing chain, which is why an RNA primer is required.)

At which “end” of the DNA strand are new nucleotides added?

New nucleotides are added to the 3' end of the strand.

Explain how DNA can be used as a template for RNA synthesis (transcription).

When RNA needs to be synthesised, the DNA is opened up (like in DNA replication), but a new RNA strand is synthesised with the help of RNA polymerase. The strand that is being copied is called the template strand. The RNA produced is complementary to the template strand. Since, due to base-pairing rules, the other DNA strand is also complementary to the template strand, this other DNA strand is known as the coding strand. The coding strand and the RNA strand have the same order of bases but the DNA coding strand has thymine in places where the RNA strand has uracil.

Explain the important features of a tRNA. What is an anticodon?

tRNA, or transfer RNA, carry amino acids to the ribosomes, where they are joined into peptides. The amino acids on tRNA are esterified to the 3' end. tRNA are roughly L-shaped and have several loops: the D-loop, the anticodon loop, the T(psi)C loop and sometimes some extra/variable loops as well. Of these loops, the anticodon loop is the most important. It contains a series of three bases that is complementary to three bases of the mRNA, allowing certain tRNA to "dock on" to appropriate places of the mRNA.

Understand how to use the triplet genetic code to “decipher” sequences of AA in proteins.

As the tRNA anticodon is made up of three bases, the mRNA is read three bases at a time. These three base sets are known as codons. Different codons code for different amino acids, though some amino acids can be coded by multiple different codons. Additionally, there are three codons (UAA, UGA and UAG IIRC) that do not code for amino acids, but instead indicate termination of transcription. (Oh and yes, there's a code for the start of transcription- it's AUG, which also codes for methionine. Sometimes this initial methionine is cleaved off during mRNA processing.) There are tables showing the "genetic code," or which mRNA codons code for which amino acids.