Chapter 8 By: Christian Borruso

In the 1950s, James Watson and Francis Crick gained insight into the possible shape of DNA. They hypothesized that it was a helix. They experimented and made many models, using scraps of metal and bonds of wire.They eventually concluded that the shape of DNA is a twisted double helix. Their work was important because many scientists built off of it in the following years.

Chargaffs first rule is that in DNA there is always equality in quantity between the bases A and T and between the bases G and C. This essentially means that in all DNA, this ratio remains the same, which leads to his second rule. DNA molecules can be hundreds of millions of nucleotides long, but the order of the base pairings can vary. These different combinations are what give us all the different kinds of life.

How do the base pairs bind?

The base pains of DNA bind with their compliment.There are two strands of DNA, and bases on one strand will bind to its compliment on the other strand. A will bind to T and G to C

A chromosome is a structure that contains DNA and the associated proteins. They carry genetic information. For most of a cell’s life, each chromosome consists of a single DNA molecule. Before cell division, it duplicates its chromosomes by DNA replication. Different organisms have different numbers of chromosomes. Humans have 46, for example.

A karyotype is an image of an individual’s diploid set of chromosomes. The karyotype differs from organism to organism. In humans, all but the last diploid are autosomes, or the same. The last one is the sex chromosome and determines the gender of the organism. It can be an x or a y.

DNA replicates itself before cell division. When the DNA starts to replicate itself enzymes break the hydrogen bonds that are within the double helix. This causes the two strands of DNA to unwind and separate from each other. A different enzyme builds primers, which are short, single strands of nucleotides. Primers act as a point of attachment for the enzyme DNA polymerase. Primers base pair with complimentary strands of DNA primer. This new base pairing is called nucleic acid hybridization. This process is driven by the hydrogen bonding. DNA polymerases attach to the hybridized primers and begin DNA synthesis. Each nucleotide provides energy for its own attachment to the end of a growing strand of DNA. Two of three phosphate groups are taken away when a nucleotide is added on to a DNA strand. The enzyme DNA ligase seals any possible gaps to make sure the new strands are continuous. Both strands are copied at the same time. As each new strand gets longer, it winds up with its template strand into a double helix. Semiconservative replication makes a total of two copies of a DNA molecule: one strand of each copy is new, and the other is parental.

Ionizing radiation from x-rays, most UV light, and gamma rays may cause DNA are all possible things that can damage DNA. They break the DNA by causing covalent bonds to form between bases on opposite strands. This alteration is very problematic because it causes adjacent nucleotide dimers to form.

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