DNA Structure
DNA Structure
Deoxyribonucleic acid (DNA) is the molecule found in all cells that provides the code that produces organisms. In eukaryotic cells, DNA is found in the nucleus, which is why the nucleus is frequently referred to as the brain or powerhouse of the cell. It is frequently called the double helix, referring to its twin strands that spiral into a helical shape.
DNA is a polymer, meaning that it is made up of repeating subunits. These subunits are called nucleotides.
There are four nucleotides in the DNA molecule. They differ by the chemical structure of the nitrogenous base that is part of the nucleotide. These bases are adenine (A), thymine (T), guanine (G), and cytosine (C).
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A and G, as pictured above are purines, which are double-ringed. T and C are
pyrimidines, which are single-ringed. When scientists were searching for the
structure of DNA, one scientist named Alex Chargoff discovered that T bonds with
A, and C bonds with G.
Rosalind Franklin took images of DNA using X-ray crystallography. These images
showed that the molecule of DNA was completely uniform.
These pieces of information among many other led Watson and Crick to build a DNA molecule out of tinker toys, always pairing A with T and G with C.
We know from the structure of the bases that A and T are always paired and C and G are always paired due to the number of hydrogen bonding sites available.
The sugars and phosphates of the nucleotides are on the outside of the helix, making the backbone of the DNA molecule.
The sugar in this case is a deoxyribose. The carbons of the deoxyribose are conventionally numbered clockwise from the oxygen peak of the sugar ring.
The two strands of DNA run antiparallel to one another. This means that while one strand has the 5' carbon of the deoxyribose up, the other has the 3' carbon of the deoxyribose up.
Cell Cycle
The cell cycle describes the activities of a cell. There are four phases in the cell cycle as shown below.
The G1 and G2 phases are known as "gap" or "growth" phases. During these phases the cell grows slightly in preparation for mitosis. Most fully differentiated cells stay in the G1 phase and do not divide. The S phase is where DNA replicates, and is therefore is known as the Synthesis phase. The M phase is mitosis. G1, S and G2 all take place in interphase (I). The G0 phase contains cells that never divide, like neurons.
Mitosis
Mitosis is cell division, where two identical daughter cells are produced from one parent cell. This process is used in most organisms for growth, maintenance, and repair. Some organisms, particularly monerans and protists, use mitosis for asexual reproduction.
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Prophase | In this phase, the DNA condenses into chromosomes, the nuclear membrane breaks down, and spindle fibers begin to form. |
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Metaphase | Here, the chromosomes line up on the metaphase plate and spindle fibers attach to the centromere. |
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Anaphase | The centromeres split and the newly separated identical strands of DNA move toward the poles of the cell. |
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Telophase | The chromosomes unwind, the nuclear membrane redevelops and the cell membrane begins to pinch off. |
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Cytokinesis | The cell membranes separate completely, leaving two identical daughter cells. |
Helpful Tutorials
Review Questions
- Adenine pairs with ____________, cytosine pairs with ___________.
- A skin cell divides to make 2 identical daughter cells in the process called
____________________.
- Pyrimidines are the ___________ - ringed nitrogenous bases in DNA and purines
are the _____________- ringed bases.
- What is complementary base pairing? Why is it important for accurate
replication?
- What is the relative strength of hydrogen bonds vs. covalent bonds? How
does this help replication?
- Explain what is meant by 2n=46.
- A nucleotide consists of 3 major parts - name them.