7Th Grade Science Notes

7 th Grade Science Notes April 10-11, 2007

DNA and RNA Structure:

. DNA (deoxyribonucleic acid) and RNA (ribonucleic acids) are both nucleic acids, one of the 4 organic polymers of life. . Both DNA and RNA have a sugar/phosphate strand (string) with nitrogen bases attached like ladder rungs between them. . The “sugar” for DNA is deoxyribose. The “sugar” for RNA is ribose. . DNA is double stranded (2 sugar/phosphate strings) with a double helix shape that can “unzip”. RNA is single stranded. . The structure of DNA was discovered by James Watson and Francis Crick in 1953 based on x-ray photograph taken by Rosalind Franklin. Unfortunately, Rosalind Franklin died of cancer shortly afterwards. Watson and Crick won the Nobel Prize in 1962 for their discovery but she never received the credit she deserved for her important help.

. Each DNA strand contains 4 nitrogen bases that form the “rungs” of the double helix ladder. They are: 1. adenine 3. thymine > IN RNA< 3. uracil replaces thymine 2. cytosine 4. guanine

. Certain“base pairs” are always together – adenine and thymine, cytosine and guanine. In RNA adenine is paired with uracil. . The abbreviation for the base pairs is their first letter : A+T and C+G for DNA; A+U and C+G for RNA. . The order in which these base pairs occur carries all the instructions for all the proteins that will ever be needed by the cell .

DNA functions:

. The chromatin that is inside the nucleus of every cell is made of uncoiled DNA. During interphase the DNA strands replicate, or make an exact copy of themselves. . Replication involves “unzipping” the DNA right down the middle of the base pairs. New base pairs attach themselves to each “unzipped” strand in exactly the same order as the original. o A-A-T-C is an example of a code along one side of the DNA strand. Make the match below:Now unzip that strand and make a copy of each. A --- ____ A --- ______----T A --- ____ A ---______----T T --- ____ T----______---A C ---- ____ C ----______---G . You now have 2 identical pieces of DNA. . Each DNA strand contains thousands of base pairs in millions of combinations to form a unique set of codes for each organism. . Once all the DNA strands are replicated, the cell enters the 2nd stage of the cell cycle – Mitosis. Now the duplicated DNA strands condense and coil into a pair of identical chromatids . The chromatids separate into each, new nucleus carrying the exact same genetic codes made of the 4 base pairs. . A genetic code is the order of nitrogen bases along a gene (section of DNA) that codes for the production of 1 specific protein. . DNA is found inside the nucleus in eukaryotic cells. Prokaryotic cells have only a single strand of DNA that floats in their cytoplasm.

Practice:

A DNA strand needs to replicate itself. It unzips and exposes its nitrogen bases. This is the sequence along one section of DNA. Fill in the other side of the strand with the letter of the correct nitrogen base.

A __ T __ C __ G __ A __ A __ C __ G __ T __ A __ G __ C __ C __ C __ T __

C __ T __ A __ G __ C __ A __ T __ A __ C __ C __ G __ T __ T __ T __ C __