Download 7th Grade Science Notes

7th Grade Science Notes
April 10-11, 2007
DNA and RNA Structure:
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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.
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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
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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 .
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DNA functions:
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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.
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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
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C
T
A
G
C
A
T
A
C
C
G
T
T
T
C
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