Mitosis and the cell cycle. DNA is replicated and this takes place during interphase.

After a cell is created it goes through a several stages:

1. G1: the first phase of growing, when proteins and organelles are being made
2. S: synthesis, when DNA is being replicated
3. G2: the second phase of growth when organelles and energy supplies are increased
4. Prophase: chromosomes become visible, nuclear envelope and nucleolus disintegrate
5. Metaphase: chromosomes line up in the middle of the cell; spindle fibres form
6. Anaphase: Spindle fibres contract pulling chromosomes to the poles
7. Telophase: nuclear envelopes and nucleoli form around both sets of DNA
8. Cytokinesis: the cells cytoplasm divides into two

1, 2, and 3 are all part of the 'interphase' when the cell is not in the process of replicating.

4, 5, 6 and 7 are all part of mitosis, which is explained in greater detail in the following post:

This diagram represents the cell cycle:

(ignore the check point markings)

ricochetscience.com

In eukaryotes, DNA is linear and associated with proteins. In prokaryotes, DNA molecules are smaller, circular and are not associated with proteins.

Eukaryote DNA

• Long chains of DNA
• Associated with proteins (wrapped up with)
• 3.2bn nucleotides long

Prokaryote DNA

• Circular chromosome
• Not associated with proteins
• 4.6mn nucleotides long

The semi-conservative replication of DNA in terms of • breaking of hydrogen bonds between polynucleotide strands • attraction of new DNA nucleotides to exposed bases and base pairing • role of DNA helicase and of DNA polymerase.

The replication of DNA is called semi-conservative because each new piece of DNA is half old DNA and half new.

1. DNA helicase breaks the hydrogen bonds between the base pairs
2. The two strands of the DNA separate, leaving 2x polynucleotide strands
3. Free activated nucleotides are attracted to the complimentary nucleotides on the polynucleotide strand
4. They are then joined together by DNA polymerase

Good to visualise with a animation: http://www.youtube.com/watch?v=zdDkiRw1PdU

Differences in base sequences of alleles of a single gene may result in non-functional proteins, including non-functional enzymes.

Bases in DNA can change (mutate) during replication.

The amino acid that the base pair coded will change, this will result in a different protein being made.

This different protein is called an allele, because it is a different form of a gene. (E.g blue or brown eyes.)

Sometimes the changes in the base pairs can make a polypeptide chain that makes a non-functional protein.

If a base is changed, one amino acid will be different which will change the protein a bit.

If a base is added in or taken away, it will change every amino acid in the chain- because it is read as in triplets so the whole sequence will move along one- which is likely to make a non-functional protein.

Enzymes are proteins, they can be made non-functional by changes in bases.

A gene occupies a fixed position, called a locus, on a particular strand of DNA. Genes are sections of DNA that contain coded information as a specific sequence of bases. Genes code for polypeptides that determine the nature and development of organisms. The base sequence of a gene can change as a result of a mutation, producing one or more alleles of the same gene. A sequence of three bases, called a triplet, codes for a specific amino acid. The base sequence of a gene determines the amino acid sequence in a polypeptide.

Three base pairs code for a amino acid. This is called the triplet code.

E.g. a thymine followed by a guanine followed by a thymine is the code for the amino acid cysteine.

A sequence of base pairs can, therefore, make a polypeptide chain (chain of amino acids that makes up a protein.)

A section of DNA that codes for a specific protein is called a gene.

Organisms are made of and controlled by proteins, so genes determine what an organism is like.

The base sequence of a gene can change as a result of a mutation,  producing one or more alleles of the same gene

An allele of a gene is a different protein produced for the same purpose.

E.g the proteins in the iris can be brown or blue (or many other colours.) So brown and blue are different alleles for the same gene.

Different alleles occur when there is a change (mutation) in the base pairs, so the amino acids are made differently resulting in a different protein.

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The locus is the place on the DNA where a gene is.

The double-helix structure of DNA, enabling it to act as a stable information-carrying molecule, in terms of • the components of DNA nucleotides: deoxyribose, phosphate and the bases adenine, cytosine, guanine and thymine • two sugar-phosphate back bones held together by hydrogen bonds between base pairs • specific base pairing

DNA is often compared to a ladder that has been twisted, this is because it consists of two back bones bonded together and then twisted into a double helix.

The back bone is made up of a sugar, deoxyribose, and a phosphate group.

These two molecules are bonded together along with an organic base; together they are called a nucleotide.

Organic bases are the molecules that make up the code of DNA.

There are four different bases: cytosine (C); thymine (T); adenine (A); guanine (G).

C and T are single ring-bases. A and G are double-ring bases (so they are twice as long.)

Single rings only ever join with double rings so that the 'rungs of the ladder' are always three rings long.

The pairing goes: C with G; A with T.

The bases are joined together by hydrogen bonds. Two for A to T; three for C to G.

• Bases code for genes
• Hydrogen bonds can be easily broken for when DNA needs to replicate
• The back bone protects the bases
• The twisting makes it smaller so more information can fit
• Covalent bonds between the phosphate and deoxyribose make the back bone strong

NB: often deoxyribose is drawn as a pentagon.

Cardiac output as the product of heart rate and stroke volume.

Cardiac output is a measurement of how much blood is being pumped by the heart, it is calculated by multiplying the speed at which it is pumping by how much it pumps each stroke.

Cardiac output= heart rate x stroke volume.