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What does DNA stand for?

Deoxyribonucleic acid


What is DNA?

DNA is the hereditary material responsible for passing genetic information from cell to cell and generation to generation.


What is DNA made from?



What are nucleotides made up of?

Three components:
A sugar called deoxyribose
A phosphate group
An organic base belonging to one of two different groups


The organic base in a nucleotide can belong to two different groups...

a) single-ring bases - cytosine (C) and thymine (T)
b) double-ring bases - adenine (A) and guanine (G)


What type of reaction are the sugar, phosphate group and organic base combined by?

A condensation reaction to produce a single nucleotide.


What is a single nucleotide called?

A mononucleotide


How are two mononucleotides combined?

As a result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another.


What is the new structure called when two nucleotides join together?

A dinucleotide


What does the continued linking of nucleotides form?

A polynucleotide


Who were the scientists who worked out the structure of DNA?

James Watson and Francis Crick


When did the scientists work out the structure of DNA?



How many strands of nucleotides is DNA made up of?

Two strands


How are the strands of nucleotides joined together?

By hydrogen bonds formed between certain bases


If DNA is thought of as a ladder which components make it up?

The phosphate and deoxyribose molecules form the uprights and the organic bases pair together to form the rungs.


Name the organic bases with a double-ring structure? (Longer molecules)

Adenine and guanine


Name the organic bases with single-ring structure? (Shorter molecules)

Cytosine and thymine


Adenine always pairs with...

Thymine by means of two hydrogen bonds.


Guanine always pairs with...

Cytosine by means of three hydrogen bonds.


The organic bases which pair are said to be...

Complementary to each other.


The quantities of which pairs of bases are always the same?

X. Quantities of adenine and thymine are always the same
Y. Quantities of guanine and cytosine are always the same
Ratios of X:Y vary from species to species.


The ladder-like arrangement is...
What is this structure called?

A double helix.


For each complete turn of the helix there are...

Ten base pairs.


How many base pairs are in the DNA of a typical mammalian cell?

3.2 billion bas pairs in one cell.


What provides the immense genetic diversity within living organisms?

The almost infinite variety of sequences of bases along the length of a DNA molecule.


How is the DNA molecule adapted to carry out its functions?

It is very stable and can pass from generation to generation without change.

The two strands are joined only with hydrogen bonds which allow them to separate during DNA replication and protein synthesis.

It is an extremely large molecule and can carry an immense amount of genetic information.

By having the base pairs within the helical cylinder of the deoxyribose-phosphate backbone, the genetic information is protected from being corrupted by outside chemical and physical forces.


Which other type of molecule did scientist suspect responsible for passing on genetic information?

The protein - chromosomes are made up of DNA and protein.


Why did scientists think that protein might be responsible for the transmission of genetic information?

Due to their considerable chemical diversity. DNA was considered to have too few components and to be too simple to fulfil the role.


What evidence was there in the 20th century to suggest it was DNA that was responsible for transmitting genetic information?

It was present in chromosomes in the right amounts
It was very stable
Its quantity halved in egg and sperm cells but not in other body cells

...But investigations had to be done to prove this.


How did people discover that it was DNA that was responsible for the transfer of genetic information?

Inject mice with i) safe living bacteria ii) harmful dead bacteria
Pneumonia caused
Explanation: the information on how to make the pneumonia toxin is transferred from the harmful form to the safe form, which then produced it.
The living harmful bacteria in mice collected and various substances isolated from these bacteria and purified.
Each substance was added to living safe bacteria - the only substance that produced this transformation was purified DNA
(when an enzyme which brakes down DNA was added, the ability to carry out the transformation ceased.)


Give another investigation into DNA as the genetic carrier...

DNA labelled with radioactive element.
Bacteria mixed with this showed signs of radioactive decay.
The process as repeated with protein instead of DNA.
There were no signs of radioactive decay.


Define the word 'gene'.

Genes are sections of DNA that contain the coded information for making polypeptides.


What do polypeptides combine to make?
What is the consequence of this...

Genes determine the proteins of an organism.


Enzymes are...



Genes dictate enzymes and hence...

They determine the nature and development of all organisms because enzymes control chemical reactions in the organism.


How many bases code for one amino acid?



How did scientist discover that three bases coded for an amino acid?

There are only 20 amino acids that occur naturally.
Each amino acid must have its own code of bases.
There are only four different bases present in DNA.

If each base coded for a different amino acid, only four different amino acids could be coded for. (4^1)
Using a pair of bases - 16 different codes are possible, which is still inadequate. (4^2)
Three bases produce 64 different codes, more than enough to satisfy the 20 amino acids. (4^3)


What is the three base code called?

The triplet code


Some amino acids have more than one...
Other do not...

Triplet code
Code for anything - they are called introns and occur within genes and as multiple repeats between genes.


Give some features of the triplet code:

The start of a sequence is always the same triplet code - for methionine. It is later removed.
Three triplet codes do not code for any amino acids - they are stop codes.
The code is universal.
The code is known as a 'degenerate code' because most amino acids have more than one triplet code.


There are no chromosomes in...

Prokaryotic cells


There are chromosomes in...

Eukaryotic cells.


When are chromosomes visible...

When a cell is dividing.


When a cell is not dividing where are the chromosomes?

Widely dispersed throughout the nucleolus.


When visible, what structure do chromosomes have?

They appear as two threads, joined at a single point.


What is each thread of a chromosome called?

A chromatid.


What is the purpose of proteins in chromosomes?

To hold the DNA in position.


How long is the DNA in a cell?



How is the length of the DNA shortened?

It is highly coiled and folded.


Why is it useful that the DNA-protein complex is coiled and then packed into the chromosome?

A lot of DNA is condensed into a single chromosome.


The number of chromosomes is always the same for...

Normal individuals of a species.


The number of chromosomes varies from...

Species to species.


In all species there is an even number of...



Why is there always an even number of chromosomes in the cells of a species?

Because the chromosomes occur in pairs called homologous pairs.


Why do chromosomes occur in homologous pairs?

Both the sperm and egg contribute a set of chromosomes to the offspring.


What are the set of chromosomes contributed by the mother called?

Maternal chromosomes


What are the set of chromosomes contributed by the father called?

Paternal chromosomes


How many homologous pairs are there in a human?



How many chromosomes are there in a human?



The two chromosomes which make up a homologous pair code for the same type of...

...genetic characteristic e.g eye colour, but they can have different informations e.g blue eyes/ green eyes.


Define 'allele':

One of a number of alternative forms of a gene. For example, the gene for the shape of pea seeds has two alleles: one for 'round' and one for 'wrinkled'.


How many alleles does an individual inherit from its parents?

Two: one from each parent.


What effect would changing the allele in a base sequence have on the polypeptide it coded for?

A different sequence of amino acids would be coded for, which would lead to a different polypeptide and hence a different protein being made. This protein could not have the infrastructure for its function which could have serious consequences for the organism.