B2 Topic 1Revision Session

Chromosome

Gene

DNA Helix

Backbone

Bases

Base pairs

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.

DNA Discovery-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.

Practice Question of DNA structure and Discover

(b) Describe the structure of DNA, including the roles of the scientists involved in its discovery.

(6)

Mark Scheme• A description including some of the following points in a logical sequence Points

relating to DNA structural features: · two strands· double helix· (contains) bases· A, T, C, G· adenine / A paired with thymine / T· guanine / G paired with cytosine / C· hydrogen / H bonds joining bases Contributions from Scientists: · X-ray (crystallography) being used· to show helical structure· to show diameter of molecule· how base pairs are arranged was shown· how strands are arranged was shown· modelling· reference to using other people's ideas

Making a Protein?

(a) Describe how a section of DNA determines the structure of a protein.

(4)

(a) Describe how a section of DNA determines the structure of a protein.(4)

• A description including the following linked points · ref to a gene (coding for protein)(1) · sequence of bases determines sequence of amino acids (1) · idea of one code / triplet / codon / 3 bases (for one amino acid) (1) · several amino acids make up a protein / (poly)peptide (1) · transcription / detail of transcription (1) · translation / detail of translation (1)

Human Genome Project

• The Idea was to map the 25000 Human genes!

• Thousands of scientists from all over the world work together.

• The big idea was to find every single gene.

• One you find them then you figure out what they do.

Good StuffPredict and prevent diseases • If doctors knew what genes predisposed people to what diseases, we call

get individually tailored advice on the best diet and lifestyle to avoid are likely problems.

Develop new and better medicinesMaybe one day we’ll all have the medicines designed especially for us- these

will be based on the way our individual body will react to the disease and to possible treatments. More generally knowing how a disease affects us on a molecular level can make it possible to design more effective treatments.

Accurate diagnosisSome diseases are hard to test for (e.g. You can only tell for sure if someone

has Alzheimer's after they die), but if we know the genetic cause, accurate testing will be a lot easier.

Improving forensic science Forensic scientists can produce a ‘DNA fingerprint’ from biological material

found at a crime science. If this matches your suspects DNA.. He or she was almost certainly present. In the future it might even be possible to figure out what the suspect looks like from DNA found at the scene of the crime.

Crops can be given extra genes for new and useful characteristics. They are genetically modified (GM).

pest resistance – wont be damaged by insects or other pests

frost resistance – wont be damaged by the cold

herbicide resistance – wont get damaged by chemicals

drought resistance – dont need as much water

longer shelf life – wont go off as quickly

disease resistance – wont get as many diseases

Producing insulin with bacteria

Why are enzymes so specific?Enzymes are very specific about which reactions they catalyze. Only molecules with exactly the right shape will bind to the enzyme and react. These are the reactant, or substrate, molecules.

The part of the enzyme to which the reactant binds is called the active site.

This is a very specific shape and the most important part of the enzyme.

What happens at the active site?In the same way that a key fits into a lock, so a substrate is thought to fit into an enzyme’s active site. The enzyme is the lock, and the reactant is the key.

enzyme

reactant

+enzyme-reactant

complex↔products

enzyme+↔

+ ↔ ↔ +

The lock and key model

• Carbohydrase or Amylase enzymes break down starch into sugar

• Protease enzymes break down proteins into amino acids

• Lipase enzymes break down fats into fatty acids and glycerol.

  Mitosis Meiosis

Purpose

To make daughter cells identical to the parent cells - eg during growth and repair

To produce sex cells (gametes)

Takes place ..In all cells apart from gametes

In the reproductive organs (ovaries and testes)

Produces how many cells?

Two daughter cells Four gametes

What happens to number of chromosomes?

Same number as in parent cellDiploid = 46 (in pairs)

Half as many as in parent cell (The original number of chromosomes is restored when two gametes fuse to form a zygote.)Haploid = 23 (single)

How do parent and daughter cells differ genetically?

Not at all - genetic material is copied exactly (replicated)

Contain a mixture of chromosomes from two parent gametes - so cannot be identical

Variation between daughter cells?

No - they are clones of each other

Yes - they are genetically different from each other because chromosomes get shuffled up during division

Mitosis and Meiosis Compared