Video Transcript
In this video, we’ll learn how a
group of scientists in the 1950s worked out the structure of DNA. We’ll also find out what genes,
chromosomes, and mutations are and why they’re so important.
Now, it’s time to hop inside our
time machine and travel back to the mid-20th century. In 1951, an English scientist
called Rosalind Franklin began investigating the structure of DNA. DNA stands for deoxyribonucleic
acid, and it’s the genetic material of all living organisms. In other words, it contains all the
information needed to make a living being, like a human. What Franklin really wanted to find
out was the structure of DNA. What does DNA look like? To do this, she carried out some
experiments in her laboratory. This involved passing X-rays
through DNA crystals and looking at the patterns that were created by the X-rays
when they came out the other side. Franklin captured the patterns as
photographs, like the one you can see here. Believe it or not, this photograph
provided a lot of evidence for the structure of DNA.
Firstly, it was determined from the
X-shaped pattern that DNA has a helical shape. Secondly, it was concluded that DNA
is made of two strands. And thirdly, it was deduced that
the nitrogenous bases of DNA face inwards towards the middle of the helix and the
phosphate groups face outwards. We’ll explain what these components
of DNA are a bit later in the video. Because this image was the 51st
photograph taken by Franklin and her team, it’s become known as photo 51 in
recognition of its significance.
In January 1953, Rosalind
Franklin’s colleague Maurice Wilkins showed photo 51 to James Watson. Watson was another scientist who,
along with his research partner Francis Crick, was trying to find out the structure
of DNA. Watson and Crick used information
from photo 51 and came up with a three-dimensional model for DNA structure. This showed DNA as having two
strands which were twisted around each other in a double helix shape, as you can see
here. Watson and Crick shared their ideas
with the rest of the world in 1953, but they didn’t mention any of the hard work
Franklin had done.
In 1962, Wilkins, Watson, and Crick
were awarded a Nobel Prize. This is a really famous award
that’s given out every year to people who have made important discoveries. By the time Wilkins, Watson, and
Crick had received their Nobel Prize, Franklin had sadly passed away from
cancer. And it was not until 1968 that
Rosalind Franklin’s significant contributions to the discovery of the structure of
DNA were properly recognized. Now, let’s look a bit more closely
at the DNA structure that Franklin, Wilkins, Watson, and Crick discovered.
DNA is made of nucleotides. Each nucleotide contains three
parts: a phosphate group, a sugar, and a nitrogenous base, which is often just
referred to as a base. As Franklin and her colleagues
found out from photo 51, the phosphate groups are on the outside of the DNA molecule
and the bases are on the inside. There are four types of base:
guanine, shown here in orange; cytosine, shown here in blue; adenine, shown here in
green; and thymine, shown here in pink. We often represent these bases with
the first letter of their names.
The DNA that we find inside every
human cell contains billions of these nucleotides. If we unwind the DNA double helix,
we can see that we have a sequence of bases on each of the two strands. For example, the sequence of bases
on this strand highlighted in red would be TCCTCGCTTGAAA. This is known as a DNA
sequence. Many of these DNA sequences contain
the information that’s needed for a certain protein to be made. In other words, the sequence of
G’s, A’s, C’s, and T’s acts as a code for that particular protein. And it’s these proteins that give
us our unique set of physical features or characteristics. For example, eye color, natural
hair color, and whether we have freckles or not are all characteristics that are
caused by DNA sequences.
A DNA sequence that determines a
particular characteristic is called a gene. As humans, we have more than twenty
thousand genes in our DNA. If we took all the DNA from a
single human cell and laid it out end to end, it would be about two meters long. So, how does all this DNA fit
inside a cell which is so tiny that we can’t even see it with the naked eye? The answer is that it’s coiled up
really tightly so that it takes up much less space. This is a bit like folding your
clothes up so that you can fit them all inside your suitcase when you go on
holiday.
Although we often talk about DNA as
if it’s one long molecule, in actual fact it’s split up into many strands, which we
call chromosomes. These chromosomes are then stored
in a specialized compartment of the cell called the nucleus. Although we’ve only shown seven
here, the nucleus of a typical human body cell actually contains 46 chromosomes. These are made up of 23 pairs as we
get half of our DNA from each of our biological parents. One chromosome from each pair is
inherited from our biological mother, and the other is from our biological
father. But what happens if our DNA
changes?
DNA isn’t lazy; it’s not just lying
around inside the nucleus doing nothing all day. It’s constantly being unwound,
read, and copied as well as being bombarded with different chemicals. These processes can cause
mutations. We often think of mutants as being
creepy-looking monsters. But in actual fact, we’re all
mutants. A mutation is just a change that
occurs in a DNA sequence. Hundreds of thousands of these
changes are happening inside us every single day. Mutations can also be
inherited. This means that when we reproduce,
we pass mutations on to our children.
Because mutations change DNA
sequences, they cause our genes to be altered. For example, can you spot how gene
x has been changed by this mutation? Hopefully, you spotted that this
thymine base has been changed into an adenine. Most of the time, mutations don’t
have any noticeable effects. But sometimes they can lead to
changes in the proteins that genes code for, which in turn can alter our
characteristics or even make us ill. For example, cystic fibrosis is a
serious disorder caused by a mutation which leads to excessive mucus production in
the lungs and the digestive system. This can make it very difficult for
sufferers of cystic fibrosis to breathe and to digest their food. Not all mutations are bad
though. Some mutations can give us
characteristics which actually improve our health or fitness. These kinds of mutations are the
basis for evolution.
Now we’ve learned all about genes
and chromosomes. Let’s have a go at a couple of
practice questions.
Which of the following scientists
was not involved in the discovery of the structure of DNA? (A) Francis Crick, (B) Rosalind
Franklin, (C) Charles Darwin, (D) James Watson, or (E) Maurice Wilkins.
Let’s remind ourselves of the
events that led to the discovery of DNA structure. In 1951, an English scientist
called Rosalind Franklin began investigating the structure of DNA. She used the technique which
involved passing X-rays through DNA crystals and then taking photographs of the
patterns that the X-rays produced. The 51st photograph, which was
taken in 1952 and has since become known as photo 51, provided a lot of important
evidence.
In 1953, Franklin’s colleague
Maurice Wilkins showed photo 51 to James Watson. James Watson was another scientist
who, along with his research partner Francis Crick, was trying to find out the
structure of DNA. Watson and Crick used information
from photo 51 to come up with their double helix model for the structure of DNA,
which they published the same year without mentioning any of the hard work that
Franklin had done. In 1962, Wilkins, Watson, and Crick
were awarded the Nobel Prize in physiology or medicine for their discoveries.
Although Charles Darwin was a
scientist who made many important discoveries about evolution, his work did not
contribute towards determining the structure of DNA. Therefore, the correct answer is
(C). Charles Darwin was not involved in
the discovery of the structure of DNA.
Let’s have a go at another
question.
Inside the nucleus, DNA is wound
and coiled into long strands. What are these strands called?
DNA is really long. In fact, if you took all the DNA
from a single human cell and laid it out end to end, it would be about two meters
long. So, how does all this DNA fit
inside the nucleus of the cell, which is so tiny that it can’t even be seen with the
naked eye? The answer is that it’s wound and
coiled to make it more compact. You can think of this process as
being a bit like folding your clothes up so that you can fit them all inside your
suitcase when you go on holiday.
Although we often talk about DNA as
if it’s one long molecule, it’s actually split up into many strands, which we call
chromosomes. Although we’ve only shown nine
here, a typical human body cell contains 46 chromosomes, and they’re stored inside
the nucleus. We have therefore determined that
the long strands of DNA that are found inside the nucleus are called
chromosomes.
Let’s summarize what we’ve learnt
in this video by reviewing the key points. The structure of DNA was discovered
by Franklin, Wilkins, Watson, and Crick. DNA is made of nucleotides. A gene is a DNA sequence that
determines a particular characteristic. DNA is arranged in long strands
called chromosomes inside the nucleus. A mutation is a change that occurs
in a DNA sequence. Mutations are usually harmless but
can sometimes cause serious disorders.
