Nucleotides are made up of:-
- A deoxyribose sugar
- A Phosphate group
- Organic nitrogenous base
There are four different nitrogenous bases in DNA:-
- Adenine pairs withs Thymine
- Guanine pairs with Cytosine
Grandma's Cottage/ Alan's Truck
Between nucleotides are hydrogen bonds.
Pyrimidine's are the bases with the letter Y, purines are the other 2 bases.
DNA introduction continued
DNA molecules are shaped in a double helix.
The strands of the helix are anti-parallel.
DNA has 2 major functions:
- Carrying information
DNA replication is important, so that daughter cells receive an exact copy of the genetic information.
DNA unwinds and unzips by breaking up the hydrogen bonds.
The breaking of hydrogen bonds is catalysed by DNA polymerase.
Each chain acts as a template which can be joined by complementary bases.
2 new DNA molecules are formed.
1 strand of each of the molecules will be conserved from the original molecule (parent)
1 strand of each of the molecules will be a newly synthesised strand (daughter)
This is called the semi-conservative hypothesis.
This was shown experimentally by Messelson and Stahl using ultra-centrifugation.
Bacteria was grown in a medium containing 15N (heavy isotope of nitrogen).
Nucleotides containing 15N were produced and centrifuged. This was found low down in the tube.
1st Generation - This bacteria was allowed to divide once - producing 2 molecules both with 1 15N strand and 1 14N strand (When centrifuged it was found in the middle of the tube)
2nd Generation - The bacteria was allowed to divide again - producing 2 15N/14N molecules and 2 14N molecules (When centrifuged it was found in the middle and the top of the tube)
DNA is found at the midpoint and the top which is conclusive evidence for semi-conservative replication
Bacteria was used because it replicates quickly and there is no nucleus.
DNA unwinds in a specific region called the cistron.
Only one strand of DNA acts as a template for the mRNA formation.
The other DNA strand rejoins to form a double helix.
Free RNA nucleotides align themselves along complimentary bases in the DNA.
The processes of DNA unwinding and the joining of nucleotides is catalysed by RNA polymerase.
mRNA carries the DNA code out of the nucleus through the nuclear pore to the cytoplasm and attaches to a ribosome.
A ribosome passes along the mRNA, (1 codon at a time)
tRNA molecule joins with an amino acid, energy is required for the process (activation)
The tRNA molecule with the appropriate anticodon attaches to the 1st codon on the mRNA.This is followed by a 2nd tRNA molecule.
A ribosomal enzyme catalyses the formation of peptide bonds form between the amino acids. The 1st tRNA molecule then leaves.
A growing polypeptide chain forms as the ribosomes move along adding 1 amino acid at a time until a STOP codon is reached.
A group of ribosomes moving along after each other is called a polysome system.
- Homologus chromosomes pair up forming bivalents
- Crossing over occurs.
- Spindle fibres appear.
- Bivalents move to the equator.
- Chromosomes attach to spindle fibres at centromeres.
- Chromosomes are pulles to opposite poles as spindle fibres contract.
- Random assortment occurs.
- Spindles dissapear.
- Chromosome number per cell halves.
- New spindle fibres develop (at right angles to old spindles)
- Chromosomes line up seperately on equator
- Spindle fibres attach to centromeres
- Centromeres divide
- Chromatides are pulled to opposite poles.
- 4 genetically different cells are produced.