Download A Unit 6 Videoscript

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Unit 6 Videoscript
Oh, man, what am I going to do? I need some real advice here. Mrs. Wanda B.
Different has commissioned me to paint a portrait of her. No problem, I’m good at that.
Only, she doesn’t like her nose that she seems to have inherited from her father. So she
wants me to use her mother’s nose, and her grandmother’s eyes, and her father’s lips.
Now, it’s okay with me because she’s paying the bill, but I think she looks good the way
she is. And it wasn’t cruel fate that gave her those characteristics, but it was random
chance combinations of the genes controlling those traits that she inherited from her
parents. Maybe I should explain some basic biology to Mrs. Wanda B. Different.
The hereditary information for all forms of life is stored in molecules on DNA that
make up genes on chromosomes in the nuclei of all cells. DNA belongs to a class of
compounds called nucleic acids. DNA itself is deoxyribonucleic acid. Here is a brief
introduction.
What they found was that the DNA molecule looks like a twisted ladder. The sides
of the ladder are made up of sugar molecules and phosphate groups repeating in a
regular pattern. The steps, or rungs, of the ladder are formed by pairs of substances
called nitrogen bases. There are four different nitrogen bases in DNA: adenine, guanine,
cytosine, and thymine. Using the work of American chemist Edwin Chargaff, Watson and
Crick determined that adenine always paired with thymine, and guanine always paired
with cytosine. Held together by weak hydrogen bonds, the base pairs form the rungs of
the DNA ladder.
Well, that was fast! Let’s go over this structure more slowly using a model. First of all,
DNA is a complex molecule formed into a shape called a double helix. She how the shape
looks like a twisted ladder? DNA is a big molecule, but it’s made of smaller parts. The
basic smaller part is called a nucleotide.
A nucleotide consists of three parts: One, a phosphate, two, a deoxyribose sugar, and
three, a nitrogen base. The sugars and phosphates make up the sides of the ladder, and the
bases make up the rungs. Here are the bases of the four kinds of nucleotides, determined
by what kind of base is present: adenine, cytosine, guanine and thymine. Let me show you
a larger model of these basic parts of DNA. These are the phosphate groups. These are the
ribose sugars that are missing one oxygen, de oxy. The four bases are adenine, thymine,
cytosine and guanine. Now, because of their structure, adenine only bonds with thymine,
cytosine only bonds with guanine. The bases and, therefore, two sides of the ladder, are
held together by relatively weak hydrogen bonds. The final structure is a double helix, like
a ladder twisted lengthwise into a corkscrew shape. So this is what a piece of DNA looks
like.
Copyright © 2008 Pearson Education, Inc. All rights reserved. Permission granted to reproduce for classroom use.
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In 1962, James Watson and Francis Crick won a Nobel prize for working out this
structure by analyzing data gathered by several other researchers. For those of you
who may appreciate the exactness of the structure, the diameter of the molecule is two
nanometers. Now, a nanometer is a billionth of a meter. The subunits are 0.34 nanometers
apart, and one full turn of the helix occurs every 3.4 nanometers.
Knowing the structure of the DNA molecule can help us answer the basic question of
how DNA can control, not only all the daily activities of each cell, but also how hereditary
information can be passed on to future generations. Research has shown us that there is
a code embedded in this molecule, and that code translates into everything a cell needs
to know. The code is the order, or sequence, of the bases on one side of the molecule.
For instance, the sequence ATC means something different that the sequence CAT or
the sequence TAG. A three-based sequence is called a codon. Now, these are all strung
together and mean something. Actually, a certain number of codons that does mean
something is called a gene. Now, you may have a gene for red hair, blue eyes, black eyes,
or a gene for the ability to roll your tongue up when you stick it out. I can’t. Some genes
may be only a few codons, or base sequences, long. Other genes may be hundreds of bases
long. The code sequence in your DNA is constantly being read in your cells. But only the
information needed at that time and place is used. So, genes switch on and off. What if
your hair color gene was working in your liver? Or the information for making toenails
was turned on as you were making earlobe cells? Hey, it’s pretty remarkable that we end
up halfway normal at all.
When cells divide, to either make a copy of themselves when you grow, or to make an
egg or sperm cell for reproduction, the DNA is able to duplicate, or replicate, itself. Hey,
take a look.
DNA duplicates, or replicates itself, so that, during cell division and gamete formation,
the cells produced by these two processes will have the necessary genetic code. In the
first step of DNA replication, the DNA molecule, which can also be compared to a zipper,
unzips. The hydrogen bonds between the base pairs are broken, and the DNA molecule
splits into two halves, or strands. Each strand of the DNA molecule serves as a pattern for
the nitrogen bases that are floating in the nucleus. Free nitrogen bases pair up with the
nitrogen bases on each strand of the DNA ladder. Adenine with thymine and cytosine
with guanine. Hydrogen bonds form between the new base pairs to complete DNA
replication. Each new DNA molecule is an exact duplicate of the original DNA molecule.
In sexually reproducing organisms, DNA from the mother and the father are passed
on to the offspring, the characteristics that the offspring show, by chance, may be those
donated by the mother or by the father.
Copyright © 2008 Pearson Education, Inc. All rights reserved. Permission granted to reproduce for classroom use.