DNA – The Molecule DNA – The Molecule Deoxyribonucleic Acid.
Molecular Genetics. What is this stuff? Simply: DNA contains the instructions for making proteins in...
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Transcript of Molecular Genetics. What is this stuff? Simply: DNA contains the instructions for making proteins in...
Molecular Genetics
What is this stuff?
•Simply: DNA contains the instructions for making proteins in a cell
• DNA = Deoxyribonucleic Acid• The genetic/hereditary
material• Referred to as a “blueprint”
for life• More of a “recipe”
DNA – The Genetic Material• DNA contains our genes:
• Gene: A segment of DNA that codes for a specific protein
• DNA is packaged with proteins to form chromosomes
• DNA wraps around 8 histone proteins nucleosomes
• Nucleosomes large, coiled loops, held by scaffolding proteins
• Loops interact Chromosomes
Why Study DNA?
• Medical purposes
• Producing better crops• Controversial, downsides
• DNA Forensics
• Evolutionary relationships
• Many, many more........
A Brief History of DNA•Hershey and Chase (1952)• Bacteriophages: attach to cell and inject own genetic material
• Cell uses genetic material to make more viruses• Labelled bacteriophage:
• Protein coat with 35S • DNA with 32P• Infect cells
•Agitate cells in a blender (shakes off bacteriophages)
• Centrifuge cells to separate from phage particles
• Only 32P found in cells, not 35S
• Conclusion: The active component of the bacteriophage that transmits the infective characteristic is the DNA. There is a clear
correlation between DNA and genetic information.
DNA Review!
•Has shape of helix or corkscrew•Is about 2 nm in diameter•2m of it in a nucleus!!•Makes a complete helical turn ever 3.4 nm•An awesome interactive review...http://learn.genetics.utah.edu/content/begin/
tour/
The Structure of DNA• DNA molecules consist of two polynucleotide chains
• The two chains, or strands, are twisted and arranged in a double-helix
• Ladder-like arrangement
• We will look at a single strand first
A Single Strand• Composed of nucleotide building blocks
• Three main components
• A pentose sugar: deoxyribose
• A phosphate
• A nitrogenous base
Turn up the Base(s)!
Structure of deoxyribose
A Single Strand• Composed of an alternating sugar-phosphate backbone: legs of ladder
• The bases stick into the middle:rungs of ladder
• The phosphate end is referred to as the 5’ (five prime) end
• Attached to Carbon 5
• The opposite end is referred to as the 3’ (three prime) end (Carbon 3)
•Consists of millions of nucleotides
DNA Is Double Stranded
• Recall the double helix
• The legs of the ladder = sugar-phosphate backbone
•The rungs = bases
• How do single strands stick together?
4 Types of Nitrogenous Bases• DNA has 4 kinds of nitrogenous bases:
• Adenine (A)• Guanine (G)• Cytosine (C)• Thymine (T)
• 2 Categories:
1)Purines: Two carbon rings• Adenine and Guanine
2)Pyrimidines: One carbon ring• Thymine and Cytosine
• Bases are attracted to each other by hydrogen bonds
• Weak, but millions of them in a DNA molecule
• This is what holds the strands together
• When bonds form:• Guanine always pairs with Cytosine – 3 bonds
• Adenine always pairs with Thymine – 2 bonds
• Base pairing is complimentary
DNA Is Double Stranded – Hydrogen Bonds
Hydrogen bonding between base pairs
DNA Is Double Stranded
• Chargaff’s Rule •1950 - Erwin Chargaff looked at percentages of bases in DNA samples
•1:1 ratio of purines to pyrimidines• Amount of G = C They always pair• Amount of A = T They always pair
DNA Is Double Stranded• Base pairing is complimentary: use one strand to predict the other
• Strands are anti-parallel: run in opposite directions
• The 3’ end terminates with the hydroxyl group of the deoxyribose sugar.
• The 5’ end terminates with a phosphate group
Finding the code...
•5’ - ATGCCGTTA - 3’
•3’ - TACGGCAAT - 5’•By convention, only the 5’ to 3’ strand is
written since the complementary strand can easily be deduced ...
•Try this one...•5’ - TGGACGCTT - 3’
•3’ - ACCTGCGAA - 5’
DNA Shape: The Double Helix• We use the analogy of a ladder that has been twisted into a helix
• Right Handed Helix: B-DNA, type generally found• Left-Handed Helix: Z-DNA
•0.34 nm distance between base pairs (10 base pairs in 1 helical turn – 3.4 nm)
•width: 2nm•Each cell has about 2 m of DNA.•How does all this DNA fit into the tiny nucleus?
• the DNA helix is coiled and long segments (200 nucleotides long) are wrapped around a complex of 8 histones (proteins)
• this bead-like structure is called a nucleosome
• DNA is negative (phosphate groups) and so is attracted to the positively charged histones
• the histones protect the DNA from breaking and tangling
•nucleosomes are packed as a solenoid, a 30 nm fiber that has at least 5 nucleosomes strands together wound helically
• this arrangement of DNA (chromatin) makes it more compact and easier to accommodate in the relatively small nucleus
• DNA (and protein) fibers are then further coiled (supercoiling) to form chromosomes
• a chromosome is one unbroken double-stranded DNA helix (only seen when they condense during cell division)
• Just before mitosis, 2 identical copies (sister chromatids) are visible, attached at a region called the centromere Packaging of DNA
V6/
Ending on a cooler note
• Each cell has about 2 m of DNA.
• The average human has 75 trillion cells.
• The average human has enough DNA to go from the earth to the sun more than 400 times.
• DNA has a diameter of only 0.000000002 m.
The earth is 150 billion mor 93 million miles from
the sun.
That’s it!
Homework...
•Answer DNA Handout Questions, Complete Build a DNA molecule Lab questions
•**DNA Extraction Lab Friday – Prelab questions p 230 of textbook.