Introduction to Molecular Genetics

What’s different from last year?

• Your genetics unit last year gave you an overview of how genes were passed on through generations, and how to use genotypes to predict phenotypes

• This year, we are going to look at how exactly that happens by studying the actual mechanisms at a molecular level

• Basically, this year, we are going to analyze how DNA as a molecule is manipulated to store and express genetic information

The basis of life• Sometime ago you saw a slide in a presentation

that summarized the flow of information in living organisms:

• DNA RNA PROTEIN

• From a molecular level, this sequence represents how information is stored in DNA and communicated by RNA to ultimately produce the end product, a protein, that serves some kind of function in the organism

Get intimate with DNA

• To better understand how all this works, we have to understand the structure of DNA to better understand it

• DNA are the blueprints for you body – each cell in your body must contain a copy of it in order to access information from these blueprints that tell the cell how to build things and how to function (we will be discussing how it makes these copies later)

• If the cell is like a construction site, the master copy of the blueprint – the DNA – never leaves the office – which, in the case of the cell, is the nucleus

Friedrich Miescher

• DNA was isolated as early as 1869, by a Swiss scientist by the name of Miescher

• He isolated DNA from pus – the collection of dead white blood cells found in infected wounds

• At this time, proteins were believed to be the basis of • He isolated what he described to be a “phosphorous

rich substance” in the nucelus of the cell that did not act like proteins

• He called this substance NUCLEIN

Hershey and Chase• In 1952 the experiment that solidified DNA as being accepted as

hereditary material involved the use of bacteriophages, viruses that infect bacteria

• Proteins contain sulfur but no phosphorous, and DNA no sulfur, but phosphorous

• Viruses are composed of a protein shell containing DNA• They tagged the viral protein with radioisotopes of sulfur and the viral

DNA with radioisotopes of phosphorous, and caused the bacteriophages (phages) to infect the E.coli, and analyzed if the radioisotopes appeared in new generations of phages created from those E.coli cells

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio21.swf::Hershey%20and%20Chase%20Experiment

Structure of DNA

• To review, DNA is composed of nucleotides

Link up• These nucleotides link up

to form a chain like structure – a single strand of DNA

A single strand• One DNA strand, therefore, is

composed of many nucleotides linked together

• The nitrogenous bases stick out from this backbone like rungs on a ladder

Free Hydrogen!

• The problem with this single strand of DNA is that it is very unstable

• The nitrogenous bases have hydrogen atoms on them that can undergo hydrogen bonding

• How can such a fragile molecule be stabilized?

Nitrogenous variety

2 Strands are stronger…

• Therefore, single stranded DNA is not feasible as a stable molecule

A twisted ladder

• DNA is described as being a DOUBLE HELIX

• Now take the ladder and twist it – the structure of a DNA molecule is very similar to this!

Molecular structure

• DNA’s structure was finalized by James Watson and Francis Crick, who relied upon Rosalind Franklin and Maurice Wikin’s x-ray crystallographic photographs of DNA to determine the overall structure

A little bit of injustice in science

• Wilkins showed Watson and Crick x-ray crystollagraphy pictures taken by Franklin and himself

• From these pictures, Watson and Crick determined the structure of DNA, and were awarded the Nobel Prize for Physiology in 1962

• Franklin died before the Nobel was given but – Franklin was given no credit for her contribution to the pictures at the time the award was given

Joachim Hammerling• Hammerling conducted a simple experiment with Acetabularia, a one celled

green algae• It was chosen because the algae had 3 regions: a foot, a stalk and a cap • The nucleus of the algae was found in the foot• It was found that when the cap was excised, a new cap would form, but if the foot

was excised, it would not regrow• He concluded that the hereditary material that instructed the plant must be

found in the nucleus – since the loss of the foot (and the nucleus) did not yield a new foot

• He also used similar species that differed in their caps and grafted different stalks to different feet – and found that the feet determined the type of cap that would regrow

• In fact, even if the whole cap and stalk was grafted onto a different foot, it would regrow the proper cap after some time!

• This further supported his hypothesis that the hereditary material was found in the foot

Fredrick Griffiths, Oswald Avery, Maclyn McCarty, Colin MacLeod

• Conducted a series of tests on Streptococcus pneumoniae that helped solidify DNA as not only hereditary material – but that it had the power to transform cells as well

• http://www.learnerstv.com/animation/animation.php?ani=17&cat=biology

http://www.johnkyrk.com/DNAanatomy.html

Opposing ends

DNA denaturing

• Because the two strands of DNA are held together by hydrogen bonds, it is worth noting that they can be separated

• In regard to our uses, this can be useful in order to manipulate DNA in laboratory experiments

Where’s the data?

• Essentially, all the information required to create you is spelled out using a four letter alphabet!

Some vocabulary• BASE PAIRS

• SEQUENCE

• GENE

• GENOME

• CHROMOSOMES

All that DNA…

• You should come to understand, after looking at the structure of DNA, that it is a rather large molecule

Expert at folding• In order to cram such a long, rope like molecule into the nucleus,

the cell winds the DNA up into tight spools of protein known as HISTONES

• Chromosome count is unique to most organisms – this is one of the reasons that species cannot reproduce outside of their own species – each species has a unique number of chromosomes that must be donated during sexual reproduction to produce a full set to ensure the genetic health of the offspring

Chromosomes

• http://www.johnkyrk.com/chromosomestructure.html

Redundant information• You would figure that more complex organisms

would require more information in their instruction manuals

• If base pairs are the “words” in your instruction manual that explain everything, then more words, more complex the instructions, right?

Our manual is as long as a mouse’s• Believe it or not, a mouse has approximately the same

number of base pairs as we do • The base pair count is not necessarily a good indicator of the

complexity of an organism – nor does it necessarily tell you everything there is to know about an organism

• For example, chimpanzees are about 98% identical to us genetically

• DNA is a complex molecule – just like reading an instruction manual – sometimes instructions will be ignored, interpreted differently by different people – in the same way, the molecule itself is not a be all and end all description of a living thing – there are many other complexities in between

The genome

• Ultimately, there base pair sequences that are redundant within a given genome

• Some are unimportant, and other parts can actually move and migrate as well

• TRANSPOSONS are sequences of DNA that can migrate to different parts of the genome

• Therefore, it is important to understand that DNA is not a static thing – it can undergo change – these changes may or may not affect the organisms depending on where they occur