12.2 The Structure of DNA
There were several people who are credited with finding the structure of this most important molecule.
Remember the 4 organic molecules (molecules that make up living things)?
carbohydrates
lipids
proteins
nucleic acids
DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.
Nucleic Acids are long, slightly acidic (hence, the name) molecules usually found in the cells nucleus (if there is one). They are polymers made up of smaller units (monomers) called Nucleotides.
A nucleotide contains 3 smaller molecules:
a phosphate group
a 5 carbon sugar (carb) called deoxyribose
a nitrogen base
There are 4 different types of nucleotides. The nitrogen bases can change but the sugar and phosphate stay the same.
The four nitrogen bases are:
Guanine
Cytosine
Thymine
Note: G and A are larger, 2 ring structures while C and T are smaller, single ring structures. Each single ring structure always pairs with a 2 ring structure in a double helix. An easy way to remember which are double ringed is that they are G and A, like GA (Georgia) which is a larger state than CT (Connecticut), C and T.
Nucleotides are held together to build nucleic acids by covalent bonding between the sugars and the phosphate groups along the outside of the "ladder".
The nucleotides can be joined in any order, meaning any sequence of bases is possible.
It just so happens that this chemical structure is especially good at absorbing UV Light. This isn't necessarily a good thing because this can cause mutations in the order of nucleotides.
Chargaff's Rule
Edwin Chargaff, an Austrian-American biochemist, had discovered that the percentages of adenine (A) and thymine (T) bases are almost always equal in any sample of DNA. The same rings true for guanine (G) and cytosine (C). This observation known as "Chargaff's rule" led us to the knowledge that T always pairs with A and G always pairs with C.
Rosalind Franklin
In the 1950's, British scientist Rosalind Franklin began to study DNA. Using x-ray diffraction, she was able to record the scattering pattern of DNA shows that the strands in DNA are twisted around each other like the coils of a spring, a shape known as a helix.
Watson and Crick
American biologist, James Watson, and British physicist, Francis Crick, had built 3D models of the molecule... but not until in 1953, when they saw Franklin's remarkable X-ray pattern, did they figure out the correct double helical structure for DNA.
Watson and Crick's model of DNA was a double helix, in which two strands of nucleotide sequences were wound around each other. This explains Chargaff's rule of base pairing and how the two strands of DNA are held together.
Antiparallel Strands
The two strands of connected DNA run in opposite directions, and are "anti-parallel".
The two strands are joined at the nitrogen bases and are held together by Hydrogen Bonds. H bonds are weaker bonds than covalent. This is helpful when the strands have to separate for DNA replication in cell division.
Note that C and G are always held by 3 H bonds, while A and T are always held by 2 H bonds.
This nearly perfect fit between A-T and G-C nucleotides is known as base pairing.
Watch Hank's great description here about the structure and replication of DNA!
For 12.2 Powerpoint click here:
https://docs.google.com/presentation/d/1w_mqwrhSKWJxE8oI4lyoN0DCpOWnX1e-2VKfEQNFiHo/edit?usp=sharing
12.3 DNA Replication
Base pairing in the double helix explains how DNA can be copied!
Each strand of the double helix has all the info needed to reconstruct the other half by using base pairing. The strands are said to be complementary.
Before a cell divides, it duplicates its DNA in a copying process called replication, which occurs during the S phase of interphase. During replication, the DNA molecule separates into two strands and then produces two new complementary strands. Each original strand serves as a template, or model, for the new strand.
During replication, the two original strands of the double helix "unzip", allowing two replication forks to form. Each strand now becomes a template for its complimentary new strand to form on. Each of the two new double strands are made up of one original strand and one new strand.
Enzymes
DNA replication is carried out by a series of enzymes.
1. Enzymes "unzip" a molecule of DNA by breaking the H bonds between the base pairs and unwinding the two strands of the molecule.
2. DNA polymerase joins individual nucleotides to produce a new strand of DNA. Then, DNA polymerase adds the sugar-phosphate bonds that hold the nucleotides together. Finally, DNA polymerase also "proofreads" each new strand to check for mistakes.
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| Micrograph showing a pair of replication forks in human DNA |
Telomeres
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| Telemeres are the white (stained) part of the blue human chromosome |
DNA at the tips of chromosomes are known as telomeres. These telomeres sometimes like to break off. This DNA is particularly difficult to replicate. Cells use a special enzyme, telomerase, to solve the problem by adding short, repeated DNA sequences to the telomeres.
Prokaryotic DNA Replication
Remember that in prokaryotes, there is no nucleus to contain the DNA. Prokaryotes have a single, circular DNA molecule in the cytoplasm, containing nearly all the cell's genetic info.
DNA replication begins when a regulatory protein binds to a single starting point on the chromosome. These proteins trigger the beginning of the S phase, and DNA replication begins.
Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. The two chromosomes produced by replication are attached to different points inside the cell membrane and are separated when the cell splits.
Eukaryotic DNA replication
Eukaryotic chromosomes are generally much bigger than those of prokaryotes. In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.
The system is not foolproof even though a number of proteins check DNA for chemical damage or base pair mismatches prior to replication.
The two copies of DNA produced by replication in each chromosome him remain closely associated until the cell enters prophase of mitosis. At that point, the chromosomes condense, and the two chromatids in each chromosome become visible.
What this video to help understand DNA replication!
Ba
For 12.3 Powerpoint click here:
https://docs.google.com/presentation/d/1pqEVHHINhKOUT1Vj8owLVpqWHmd8PaYE13Dwtb10hBg/edit?usp=sharing
