Palindromic Sequences

Restriction enzymes cut double-stranded DNA* at specific locations based the pattern of bases found at those locations. These enzymes predictably cut both strands because the sequences they recognize are palindromic. That is the recognition sequences are short string of identical bases on both DNA strands.

Palindromic sequences are similar to language palindromes, but follow a distinct set of rules. Any string of bases can be made into a palindromic sequence by following these rules.

Restriction Enzymes

Restriction enzymes cut DNA* at specific sites based on the sequence of bases along the strand at the cut site. These enzymes were first identified and studied in strains of the bacteria E. Coli in the 1950’s and 60’s. The term restriction was used to describe them because their activity restricted the growth of viruses that infect E. coli.

Restriction enzymes are nucleases - enzymes that cut nucleic acid polymers (i.e. DNA and RNA). There are two types of nuclease: endonuclease and exonuclease. Endonucleases make cuts within a DNA polymer. Exonucleases remove individual nucleotides* from the end of a strand. Restriction enzymes are a type of endonuclease - they cut at specific sites in the middle of DNA strands.

Gel Electrophoresis Overview

Electrophoresis is the movement of charged particles through an electrical field. Since the sugar-phosphate backbone of DNA

Meselson–Stahl Experiment

In their second paper on the structure of DNA*, Watson and Crick (pdf) described how DNA's structure suggests a pattern for replication:

"…prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains, where we only had one before." - Watson and Crick, 1953

This is called semiconservative replication.

Today we know that this is the pattern used by living cells, but the experimental evidence in support of semiconservative replication was not published until 1958. In the 5 years between Watson and Crick's suggestion and the definitive experiment, semiconservative replication was controversial and other patterns were considered.

Drift and Selection

The Hardy-Weinberg equation describes allele frequencies in populations. It predicts the future genetic structure of a population the way that Punnett Squares predict the results of an individual cross. The equation calculates allele frequencies in non-evolving populations. It is based on the observation that in the absence of evolution, allele frequencies in large randomly breeding populations remain stable from generation to generation.


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