DNA,RNA, and Proteins

In the 1950’s, James Watson and Francis Crick, built a model of DNA. Their model was inspired by the work of Rosalind Franklin and Maurice Wilkins who had made x-ray photographs of DNA crystals.

Watson and Crick

Rosalind Franklin

The model proposed by Watson and Crick, not only explained the structure of DNA but also showed how DNA was able to replicate. You may remember that whenever cells divide, the DNA must replicate(make a copy of itself) During S phase

DNA is made of two long chains Each chain is made of repeating units called nucleotides. Each nucleotide is made of : 1. a sugar 2. a phosphate group 3. a nitrogenous base

The sugar in DNA is deoxyribose sugar The phosphate group contains a phosphorus atom and four oxygen atoms. There are four kinds of nitrogenous bases in DNA

Two of the bases are called purines because they have a double ring of carbon and nitrogen atoms. The purines are adenine and guanine.

Two of the bases have just one ring of carbon and nitrogen atoms. They are called pyrimidines. The pyrimidines are thymine and cytosine.

Watson and Crick determined that the DNA molecule was a double helix in which the sides of the ladder were made of alternating sugars and phosphates and the nitrogenous bases formed the rungs. The rungs were always made of one purine and one pyrimidine so that they are all the same width.

Purines, Pyrimidines,and Nucleotides

Erwin Chargaff had already observed that the percentages of adenine and thymine are always equal in This meant that adenine always joined to thymine and cytosine always joined to guanine. This is important because these base pairing rules provide a way for DNA to replicate.

During S phase, enzymes cause the DNA to unzip. Other enzymes help each half of the DNA ladder to pick up nucleotides to replace the missing half. Because of the base pairing rules, this results in two identical molecules of DNA. Replication

DNA Replication

Although DNA has all of the instructions for building your proteins, DNA can’t actually make them. The proteins are all made at the ribosomes which are out in the cytoplasm. DNA never leaves the nucleus. This means that DNA needs a messenger.

DNA makes a copy of its information in the form of a different chemical called RNA. RNA stands for ribonucleic acid RNA can leave the nucleus.

RNA is also made of nucleotides. But: the sugar is ribose RNA does not contain thymine and replaces it with uracil RNA is not a ladder. It is one strand instead of a double strand

RNA

DNA makes RNA in a process called transcription. Transcription means to copy something. Transcription is similar to the process of replication. The DNA unzips and enzymes help the DNA to pick up nucleotides to replace the missing half Transcription

The difference is that only one half of the DNA is copied. This is important because if it used both halves you would get RNA with two different sets of instructions. The other difference is the nucleotides conatin ribose instead of deoxyribose and uracil instead of thymine.

There is more than one type of RNA. One type is called messenger RNA or m-RNA. m-RNA carries the instructions for building proteins to the ribosomes.

Translation Proteins are built by connecting amino acids. There are 20 different amino acids. Each sequence of three nucleotides is the code for one amino acid. Example: GCU is the code for alanine but UGG is the code for tryptophan.

A sequence of three nucleotides on messenger RNA is called a codon. There is another kind of RNA called transfer RNA or t=RNA. Transfer RNA molecules pick up the amino acids and take them to the m- RNA.

Transfer RNA

Each transfer RNA molecule can only pick up one kind of amino acid. Each transfer RNA has an anticodon which fits the codon on the m-RNA. This is how the amino acids get placed in the correct order.

Changing the genetic code into proteins is called translation. Translation takes place at the ribosomes

Translation

Genetic Code