Download F Unit 2 Videoscript

F
Unit 2 Videoscript
How do generations differ from one
another?
What these animals have in common is obvious; they’re all part of the same family. But a
closer inspection reveals that they are also different from one another. The reasons animals in the
same family are different and the reasons that one type of animal is different from another can
be found in a fascinating field of science known as genetics, the study of heredity. All living things
resemble their parents, but each individual also has certain unique characteristics, or traits.
Can you think of some traits that make people different from one another?
Traits like eye color, hair color, skin tone, height and right or left-handedness are just a few
of the ways that we’re different from one another. Discovering how and why this happens is a
fascinating journey, one that was first undertaken by a nineteenth-century monk and teacher
named Gregor Mendel.
Working in the garden of a small monastery in Austria, Mendel conducted experiments and
made observations on pea plants. Mendel observed and experimented on several traits of pea
plants, including how tall the plants grew, and the color and shape of their seeds. By keeping
careful records of his experiments, Mendel was able to piece together the basic laws of heredity.
Mendel discovered that each trait he studied was influenced by a pair of factors now called
genes. Genes are the basic unit of heredity and, in organisms that reproduce sexually, each
parent contributes one gene per each trait to their offspring. In experiments involving the traits
of tallness and shortness, he also discovered that one trait is stronger, or dominant, and the
second is weaker, or recessive.
A dominant trait is expressed whenever its dominant gene is present. But a recessive trait
seems to disappear whenever a dominant gene is present. In other words, dominant genes mask
the presence of recessive genes. Recessive traits are expressed only when both genes for a trait
are recessive.
A few years after Mendel’s work was published, a German botanist by the name of Carl Correns
added to Mendel’s work on dominance when he discovered incomplete dominance. Working with
four o’clock flowers, Correns noticed that, in some gene pairs, neither gene is dominant, and the trait
expressed by the gene pair appears to be blended; a combination of the two traits.
Gregor Mendel presented his findings to a meeting of scientists in 1865. He was very
disappointed that his work went unnoticed, mainly because the scientific world was focused on
Charles Darwin’s Origin of Species. But it was Mendel’s identification of dominant and recessive
genes that helped to explain and clarify how Darwin’s theory of natural selection could bring
about change in the species.
Copyright © 2008 Pearson Education, Inc. All rights reserved. Permission granted to reproduce for classroom use.
F
Mendel is considered the father of genetics, but with the greater understanding of the
function of different parts of the cell, and new tools for research, many others have followed
in his footsteps.
Gregor Mendel’s experiments began to unravel the mysteries of heredity, but he left some
major questions unanswered, like, what are genes made of? And what carries the genetic code
from one generation to another? How does DNA store, duplicate and pass on the genetic code?
The answer, many felt, would prove to be the most important of all scientific discoveries,
and the race was on to find it. Groups of scientists worked feverishly to uncover one of
science’s greatest mysteries. But when the answer came, it began with a very simple statement.
We would like to suggest a structure for the salt of Deoxyribose Nucleic Acid. This structure
has novel features, which are of considerable biological interest.
The words were those of American biologist James Watson and British biophysicist
Francis Crick.
And in just nine hundred words, the young duo proceeded to answer the question that
scientists the world over had been seeking.
What was the structure of DNA and how does DNA work?
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 biochemist 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.
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 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.
Geneticists have come a long way towards giving us an accurate picture of how life
operates. While nobody can predict where their research will lead, one thing is for sure—
our understanding of genes, those infinitely lengths of DNA that determine the nature of
life itself, is growing rapidly.
Copyright © 2008 Pearson Education, Inc. All rights reserved. Permission granted to reproduce for classroom use.