Wednesday, 26 February 2014

The importance of meiosis in producing cells which are genetically different. Within this unit, meiosis should be studied only in sufficient detail to show • the formation of haploid cells • independent segregation of homologous chromosomes. Gametes are genetically different as a result of different combinations of maternal and paternal chromosomes • genetic recombination by crossing over.

Meiosis is the process by which sex cells (gametes) are produced. They have half the genetic information of a normal (somatic) cell.

The 'normal' amount of chromosomes is called the diploid number (in humans it is 46 chromosomes/23 pairs of chromosomes) and gametes have the haploid number of chromosomes (23 in humans).

It is important that gametes have half the genetic information so that when two gametes meet the child gets a mixture of genes from its parents and therefore is very different. This is called variation and is what has enabled evolution to happen.

Haploid cells are made in two stages: meiosis 1; and meiosis 2:

Prophase 1: chromatin condenses to form chromosomes. Homologous pairs (the same chromosomes, one from your mum and one from your dad) come together, here some segments of DNA can transfer from one into another, so alleles will swap- this is called crossing over and increases variation. Nuclear envelope and nucleolus dissolve. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 1: homologous pairs line up in the middle of the cell, some of the paternal chromosomes are on the right and some on the left, the same for maternal chromosomes.

Anaphase 1: the pairs are pulled apart towards the opposite poles by spindle fibres.

Telophase1 : the nuclear envelope and nucleus reform (and cytokineses happens) so two new cells have been made. Because some of paternal and maternal DNA was in a random order, the cells have different DNA, this is called independent segregation.

Meiosis 2 happens in both of the cells formed by meiosis 1, it is the same as mitosis:

Prophase 2: nuclear envelope and nucleolus dissolve. Spindle fibres start to form. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 2: chromosomes move into the middle.

Anaphase 2: spindle fibres pull sister chromatids apart in towards opposite poles.

Telophase 2: nuclear envelope and nucleolus reform (and cytokineses happens) so that each cell has become two, resulting in four cells, each with half the number of chromosomes than a somatic cell.


Candidates should be able to analyse, interpret and evaluate data concerning early experimental work relating to the role and importance of DNA.

One experiment performed when investigating the nature of DNA was one to find out if its replication is conservative or semi conservative.

Conservative means there would be one completely new piece of DNA made and one completely old one left.
Semi-conservative means there are two pieces of DNA that are both made up of half old and half new DNA.

DNA contains nitrogen.
There are two isotopes of nitrogen 14N and 15N.
15N is heavier and 14N lighter.
If DNA contains 15N it would sink towards the bottom of a tube when centrifuged.
If DNA contains 14N it would be towards the top of a tube when centrifuged.

Scientists grew bacteria on 14N, they then removed the DNA and centrifuged it- it came out at the top of the tube, showing that the DNA of the bacteria would have in it the DNA it grew on.

Scientists then grew bacteria on 15N. This DNA came out at the bottom of the tube.

Scientists moved the bacteria from the 15N to the 14N and left them long enough that the DNA would replicate once.

They found that DNA from these bacteria came out in the middle of the tube. This was because they had one light strand, and one heavy strand. This showed that the DNA included one old strand of DNA from when it was on 14N and one new strand of DNA from when it was on 15N.

So they could conclude that DNA replicated semi-conservatively, making two strands of half-new-half-old DNA.

Tuesday, 25 February 2014

DNA and chromosomes resources

Mitosis resources

I've decided to put up resources that I use to create posts and revise, so enjoy:

During mitosis, the parent cell divides to produce two daughter cells, each containing an exact copy of the DNA of the parent cell. Mitosis increases the cell number in this way in growth and tissue repair. Candidates should be able to name and explain the events occurring during each stage of mitosis. They should be able to recognise the stages from drawings and photographs.

We always need more cells (to replace old ones and) for growth and tissue repair.

In mitosis, one cell divides into two new cells: the first cell is the parent cell and those produced are daughter cells. The daughter cells are identical to the parent cell because they have the exact same DNA.

There are four phases in mitosis:

DNA that was in chromatin form to condenses into chromosome form: that means it associates with proteins and winds itself into a structure. Each chromosome consists of two chromatids (identicle pairs of DNA) which are joined in the middle by centromeres.
Centrosomes move towards the poles (opposite ends) of the cell and begin to form spindle fibers (micro tubules that act like rope).
The nuclear envelope and nucleolus dissolve.

Chromosomes line up in the middle of the cell.
Spindle fibres which attach to the centromere of chromosomes.

Spindle fibres pull one half of each chromosome in each direction, at the same time the centrosomes are moving further apart towards the poles of the cell.

Chromosomes reach poles and turn back into chromatin.
The nuclear envelope and nucleolus reform.

Candidates should be able to relate their understanding of the cell cycle to cancer and its treatment.

If there is a change in the DNA that controls the cell cycle, there can be an increase in the speed that cells grow at.

One cell with this mutation will rapidly divide into to, and this will continue to create a large group of cells, called a tumour.

These tumours are potentially disruptive to the functions of organs, as we know cancer takes many lives.

Chemotherapy is the use of chemicals to inhibit the cell cycle. They effect parts of the cell cycle for example: stopping spindle formation in the metaphase; stopping DNA replication during synthesis.

These chemicals effect the cycles of all cells in the body, but they will have a bigger effect the faster the cycle. This means that cancer cells are significantly slowed, but so are other quickly replicating cells are too, like hair cells.

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)

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

Thursday, 20 February 2014

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:

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.


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.