DNA

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					              Dr. Childs
DNA   Science TAKS Lab
              Fall 2004
Why does a mother dog have puppies instead of
kittens? Why do we look like our parents? The
answer is "genes". Half of our genes are inherited
from your mother and half from your father.

  “He’s inherited my good looks!”
Genes code for traits. Traits include:

     - color of kernels of corn
     - our brown eyes
     - a puppy’s floppy ears
But what is the nature of a "gene"? How is
information for brown or blue eyes, straight
or curly tails, or yellow or black corn kernels
transmitted. The secret is in the molecule -
DNA.
It is important that we understand three processes:


Replication -     how the DNA replicates

Transcription -   how information on genes is
                  coded to a messenger RNA

Translation -     code on the messenger RNA is
                  used to express proteins.
We will also learn how changes in this code
may contribute to mutations which lead to
cancer or sickle cell anemia or, perhaps, a
white tiger.
The genes that carry traits from one generation to
the next are located in the molecule –
deoxyribonucleic acid or “DNA”.

DNA is a macromolecule that is shaped as a
double helix.
The double helix of DNA is composed of units
called nucleotides.

Nucleotides are composed of three parts:

     - 5-carbon sugar (deoxyribose)
     - phosphate group
     - one of four nitrogen bases
DNA, “deoxyribonucleic acid“, is named
after the 5-carbon deoxyribose sugar.
The four nitrogen bases pair in DNA:

     thymine (T)         -     adenine (A)
     guanine (G)         -     cytosine (C)
Important: uracil (U) substitutes for thymine (T) in RNA
The structure of DNA
may be compared to a
ladder:

Sides = sugar–phosphate

Rungs = nitrogen bases
                  Complementary Pairing

Bases on one strand are complementary to the other strand.
Strands are also defined as complementary.

       adenine    (A)   -   thymine    (T)
       thymine    (T)   -   adenine    (A)
       cytosine   (C)   -   guanine    (G)
       guanine    (G)   -   cytosine   (C)

Important:
                  A–T
                  G-C
                   Complementary Pairing


You will probably see 3' and 5' on strands of DNA on TAKS
test. This notation refers to a direction (3' -> 5') of a strand.
Complementary strands goes in opposite directions.

3' A - T - T - G - C - A - G - G - A - A - T - C 5'

5' T - A - A - C - G - T - C - C - T - T - A - G 3'
            Replication

Replication is the copying of a DNA
  molecule

1. DNA unwinds;

2. Each old strand serves as a
   “template” for a new strand;

3. New complementary bases added
   to each template; and

4. Two double helixes separate. Each
   has on old strand and a new
   strand – “semiconservative
   replication”
Genes code for the sequence
of amino acids that make up
proteins!
                       Proteins
There are possible thousands of different proteins with a
wide range of functions. When we look at each other what
we see are proteins (skin and skin pigments). These
include such proteins as:

    keratin             skin, hair
    melanin             dark skin pigment
    hemoglobin          carries oxygen in red blood cells
    enzymes             regulate cellular processes
    antibodies          attack bacteria and viruses
                       Proteins
Proteins are chains of amino acids. There are 20 different
amino acids. The test may use common three letter
abbreviations. For example:

      Aspartine            Asp
      Glycine              Gly
      Histidine            His

DNA codes for specific amino acids by the sequence of
bases. The sequence of amino acids codes for specific
proteins.
Protein Synthesis – Cell Structures

                   DNA is located in
                   chromosomes in the cell
                   nucleus

                   Ribosomes are the
                   “machines” in the cytoplasm
                   on which proteins are
                   assembled
                         Gene

A gene is that it is the sequence of nitrogen bases in
  DNA that code of a single protein.




         (Whoops, wrong Gene)
                 Genes to Proteins

Protein synthesis requires an understanding of:


 ribonucleic acids a second class of nucleic acids (= RNA)

 transcription     process of transferring the code from DNA

 translation       synthesis of protein at ribosome
            Comparison of DNA & RNA

                     DNA                    RNA
sugar      deoxyribose          ribose

bases      adenine - thymine    adenine - uracil
           guanine - cytosine   guanine - cytosine
shape      double helix         linear

location   nucleus              cytoplasm

function   genes                protein synthesis
              Ribonucleic Acids (RNA)
There are three classes of RNA with different functions:

messenger RNA (mRNA)
   copies the codes from the DNA gene and carries it to
   the ribosome

transfer RNA (tRNA)
    specific tRNA carries amino acids to the ribosome for
    protein synthesis

ribosomal RNA (rRNA)
    ribosomes are composed of rRNA and proteins
                  Messenger RNA


messenger RNA (mRNA)

- copies the code from the genes in the DNA

- carries the code through the membrane of the nucleus
        into the cytoplasm where it codes for the assembly
        of proteins
                        Codons
Information on mRNA is coded in three letter “words” called
“codons”.

Each codon corresponds to a specific amino acid. For
example CAU codes for the amino acid “Histidine” or “His”

There are also codons for “start” and “stop”.
                   Transcription
- Code on DNA is copied to mRNA.
   note: “U” on mRNA is complementary to “A” on DNA

• mRNA leaves nucleus through nuclear pore.
                     Translation
-Translation occurs on ribosomes. Amino acids brought to
       ribosome on specific tRNA. “Anticodon” on tRNA
       complementary to “codon” on mRNA.

-Protein assembled one amino acid after another.
                     Mutations

A mutation is a change in the base sequence of the DNA.

This may be caused by a mutagen, something that causes
a mutation. For example:

      -   UV rays from the sun
      -   X-rays
      -   smoking
      -   some chemicals as asbestos
                       Mutations
Many mutations are harmful.

A mutation to a control region of a gene may cause a cell
to multiply without stopping and become cancerous. Tars
in cigarette smoke are mutagens that may cause cancer.

Sickle cell anemia is a mutation of a single base in the
gene that codes for hemoglobin.
                        Mutations
But not all mutations are harmful.

Mutations that lead to variations in populations that allow
plants or animals to adapt to changes in their
environment.

For example, carriers for sickle cell anemia have
protection against malaria.
                       Mutations
Mutations may different effects depending on whether the
mutation is in a reproductive cell (egg or sperm) or another
body cell. For example,

UV rays may cause skin cancer. These are not inheritable.

However, X-rays to the ovaries or testes may cause
mutations to egg or sperm cells which may be inherited.


                                         Lead aprons prevent
                                         exposure of reproductive
                                         organs to X-rays during
                                         dental exams
                       Mutations
Mutations may occur at the level of a single base in the
DNA. Depending on the mutation there may be no change
in the protein sequence. However, there could be a multiple
amino acid change or an abrupt shortening of the protein is
a codon is mutated to a “stop codon”
                            Mutations

Almost every TAKS test has a question involving a mutation. It may be
related to the effects on a protein due to the change of a single
base/codon on the mRNA.


This may have one of three results. For example:

   UAU to UAC (Tyr to Tyr) would have no effect

   UAU to UCU (Tyr to Ser) would change a single amino acid

   UAU to UAA (Tyr to STOP) would cause the protein synthesis to stop
“Stop” codon

				
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