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Lesson Video: Genes and Chromosomes Science

In this video, we will learn how to describe the structure of chromosomes, and explain what a gene is.

11:30

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.

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