DNA / RNA

Notes

l. DNA Structure A. Chromosomes are made up of

DNA, or deoxyribonucleic acid. DNA is the master copy, or blueprint, of an organism’s information code.

B. DNA is a type of nucleic acid, which are made up of subunits called nucleotides.

C. A nucleotide consists of 3 things:

A sugar- deoxyriboseA phosphate groupA nitrogenous base. In DNA, there are 4 bases: adenine, guanine, cytosine, & thymine.

D. DNA looks like a twisted ladder. This structure, called a double helix, was proposed by Watson & Crick in 1953.

E. The “poles” of the ladder are made up of alternating sugars and phosphates. The 2 poles are held together by nitrogenous bases (the “steps” or “rungs” of the ladder).

1. The nitrogenous bases are held together by weak hydrogen bonds.

Nitrogen basesPoles of sugars &

phosphates

2. The bases pair very specifically:

•- Adenine only pairs with Thymine

•- Guanine only pairs with Cytosine

A – T G - C

F. The way the nucleotides are arranged on the double helix forms a genetic code. The cell can “decode” this code in order to make proteins. Proteins determine the structure and function of organisms.

ll. DNA replication or copying

A. Every time a cell divides, it must first make a copy of its chromosomes, or DNA. This process of copying is called DNA replication.

B. How does it happen?1. An enzyme breaks the hydrogen bonds that hold the 2 strands together, unzipping the DNA.

2. This leaves 2 single strands of DNA with unmatched/ unpaired bases.

3. Free nucleotides in the surroundings pair to the exposed bases.

4. Each new strand is a complement of one of the original, or parent, strands.

5. The result is the formation of 2 DNA molecules, each identical to the original.

lll. RNA and Protein Synthesis

How does a cell use the info in DNA to make proteins?

A. Parts of the DNA sequence are copied into RNA, or ribonucleic acid. The RNA molecules carry out the process of making proteins.

B. RNA structure vs DNA structure

1. RNA is single stranded 2. The sugar in RNA is ribose 3. RNA contains uracil instead

of thymine

C. Transcription: Making RNA from DNA1. Messenger RNA (or mRNA) is a “copy” of DNA that can travel from the nucleus to the ribosome, where proteins will be made.

2. We will talk about bases in 3’s, what is known as a triplet codon.

lV. Translation A. Proteins determine the structure and

function of organisms. They are made of amino acids. Only 20 amino acids occur naturally, but these 20 can be arranged to produce an infinite number of proteins.

B. Translation is the process of converting the sequence of bases in mRNA into a sequence of amino acids, making a protein.

C. Steps:1. mRNA attaches to a ribosome.

2. tRNA (or transfer RNA) brings amino acids to the ribosome so that they can be assembled into proteins. How does the tRNA know where to bring the amino acid?

3. At the bottom of the tRNA are 3 nitrogen bases known as the anticodon. The tRNA looks for the complementary codon and attaches the amino acid at that spot.

4. The process continues and a chain of amino acids is formed. The amino acids bond to form a protein.

V. Mutations A. Sometimes mistakes occur in

the process of copying genetic material. Any mistake or change in the DNA sequence is known as a mutation.

B. Point mutations- a change in a single base pair in DNA. This change can lead to a change in amino acids, and therefore, a change in a protein.

C. Frameshift Mutation: A single base is added or deleted, changing every amino acid after the change.

AUG – ACC – AUU – ACG – GAU Met Thr Ile Thr Asp

AUG – ACC – ACG – GAU Met Thr Thr Asp

D. Chromosomal Mutation: ex. Parts broken off. Gametes may end up with extra copies of some genes or a lack of some genes.

E. Disjunction errors: Failure of homologous chromosomes to separate properly in meiosis is called nondisjunction. Two kinds of gametes can result:

1. Extra chromosome 21: Down’s syndrome

2. Lack of chromosomes: zygotes usually die

G. Causes of mutations1. Random/ Spontaneous2. Environmental- x rays, UV, certain chemicals, etc.