DNA and inheritance and the differences between organisms by CGP

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DNA Inheritance

DNA's brilliant stuff- it's like your body's own instruction manual. When you're being made, you get bits of DNA from your mum and bits from your dad- this is how you inherit characteristics.

Chromosomes, DNA and Genes

Most cells in your body have a nucleus. The nucleus contains Chromosomes

Chromosomes are long, coiled up lengths of a molecule called DNA.

DNA is a long list of chemical instructions on how to build an organism.

A gene is a short section of chromosomes (and so a short section of DNA)

Genes control many of our characteristics

Genes work in pairs- one will usually be dominant over the other

We Inherit Charisteristics from our parents

Human body cells have 46 Chromosomes (23 pairs)

Sperm and egg cells carry only 23 chromosomes

During reproduction, when an egg is fertlised,the nucleus of the egg fuses with the nucleus of the sperm

This means that the fertilised egg contains 23 matched pairs of chromosomes. It has one copy of each gene from the mother and father

Since genes ontrol Charisteristics, the fertilised egg developes into an embryo with a mixture of the parents Charisteristics. This is how you inherit your parents' Charisteristics

This process by which genes are passed down from parents to their offsprings is called heredity

The structure of DNA was only worked out recently

Scientists struggled for decades to work out the structure of DNA.

Crick and Watson were the first scientists to build a model of DNA- they did it in 1953

They used this data from other scientists, Wilkins and Franklin, to help them understand the structure of the molecule. This included X-ray data showing that DNA is a doble Helix- a spiral made of two chains wound around together

By putting all the information together, Crick and Watson were able to build a model showing what DNA looks like.

Variation

This is about the diffrences between organisms - both big, obvious defences, like those between a Tree and a cow, and less obvious differences, like people having different blood groups

Different Species have different genes

All living things in the world are different- we say that they show Variation

A human, a cow, a dandelion and a tree all look different because they have different species. These differences between species occur because their genes are very different

But you also see a variation within a species i.e plants or animals they have basically the same genes will also show differences between them, e.g skin colour, height, flower size ect. Any differences are known as a Charisteristic features

Charisteristic features can be inherited or they can be environmental

Continuous and Discontinuous Variation

Variation within a species can either be classed as continuous or discontinuous

Continuous Variation- The features can vary over a range of values

Examples of this are things like height, weight, skin colour, intelligence, leaf area, ect. Where the feature can have any value at all- within a certain range. If you did a survey of kids' heights you could plot the results on a chart like the one opposite (the heights would be collected into groups to give the bars)

The smooth distribution curve drawn on afterwards shows much better the continuous way that values for height actually vary.

Discontinuous variation- The feature can only take care of certain values

An example of this is a person's blood group, where there are just four distinct options, not a whole continuous range