Specification Link

• Recall that a gene is a section of a molecule of DNA and that it codes for a specific protein

• Describe a DNA molecule as:a) two strands coiled to form a double helixb ) strands linked by a series of complementary

base pairs joinedtogether by weak hydrogen bonds:i adenine (A) with thymine (T)ii cytosine (C) with guanine (G)

• Explain how the structure of DNA was discovered, including the roles of the scientists Watson, Crick, Franklin and Wilkins

What would Mr Singh’s and Mila Kunis’s Baby look like?

Big Question

What is DNA and how was it discovered?

In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents.

Passing on characteristicsWhat makes this baby human? What determines its gender?

This means that all human characteristics must be something to do with chromosomes. Where are chromosomes found?

Inside the nucleus….

What’s this?

Chromosomes are long strands of genetic information located in the nuclei of cells.

Chromosomes are most visible during cell division when they replicate and look like this…

What do chromosomes look like?

Most cells of the body contain chromosomes in matching pairs. These pairs are called homologous chromosomes. Where do homologous chromosomes come from?

chromosome from female parent

chromosome from male parent

homologouschromosomes

Homologous chromosomes

Each pair of homologous chromosomes contains one chromosome that has been inherited from each parent.

What we should know..

• In the nucleus there are 23 pairs of chromosomes (half from mum, half from dad).

• Chromosomes contain genes. • The instructions to make you is found

in the nucleus.

But what exactly are chromosomes?

DNA genes

chromosomesContains the

Nucleus of

every cell

Contains the

Made of

What is DNA?

Chromosomes and their genes are made of a molecule called DNA.

DNA molecules carry the code that controls what your cells are made of and what they do.

DNA stands for deoxyribonucleic

acid.

Each chromosome is a very long molecule of tightly coiled DNA.

Which part of a DNA molecule holds this information?

The structure of DNA

Chromosome

Gene

DNA Helix

Backbone

Bases

Base pairs

DNA and base pairs

Base pairs hold the two strands of the DNA helix together.The rules for base pairing are…

There are millions of base pairs in a DNA molecule that always follow these rules.

Adenine always pairs with Thymine

Cytosine always pairs with Guanine

Amazingly, it is the sequence of bases along a DNA molecule that forms the genetic code – it’s that simple!

The Bases

Base pairs hold the two strands of the DNA helix together.

The sequence of these bases along a DNA molecule forms the genetic code

Phosphate Group

Hydrogen Bonds Deoxyribose

DNA Structure Key Points

• Each gene is a length of DNA, DNA is a long coiled molecule formed from two strands. The strands are twisted in a double helix.

• The two strands of the double helix are joined by pairs of bases. There are four different bases in DNA.

• A= Adenine, T=Thymine, C=Cytosine, G=Guanine

• Weak hydrogen bonds between the base pairs hold the DNA strands together.

• Modelled in 1953 by James Watson & Francis Crick.

• Awarded Nobel Prize in Physiology or Medicine in 1962 to Watson, Crick and Wilkins.

Franklin’s “Photograph 51”

Watson & Crick’s ball and stick model

• Experiments carried out in the first half of the twentieth century led scientists to the view that DNA was the genetic material that contained the instructions for life (not proteins as had been previously thought).

• Erwin Chargaff (1905–2002), working in the USA, analysed the components of DNA taken from different organisms – animals, plants and bacteria. He measured the proportions of adenine, guanine, cytosine and thymine. Some of his results are shown in the next slide.

%A %G %C %T

maize 26.8 22.8 23.2 27.2octopus 33.2 17.6 17.6 31.6chicken 28.0 22.0 21.6 28.4wheat 27.3 22.7 22.8 27.1human 29.3 20.7 20.0 30.0

Key Points

• Rosalind Franklin and Maurice Wilkins worked out that DNA had a helical structure by directing beams of X-rays onto crystallised DNA and looking at the patterns the X-rays formed as they bounced off.

• James Watson and Francis Crick used these ideas along with the knowledge that the amount of adenine +guanine matched the amount of thymine +cytosine, to make a model of DNA molecule where all the pieces fitted together.