Properties expected of genetic material
Accurate replication during cell growth and duplication
Stable structure so that hereditable changes can occur rarely (but are possible)
Have the potential to carry all of the necessary biological information
Can transmit information to the cell
Location - Hammerling & Brachet
Hammerling and Brachet (1930s-1940s) studied Acetabularia.
It is a small seaweed, single celled, 5cm high and had 3 cellular regions; cap, stalk and holdfast.
There were two species which were used:
- Acetabularia mediterranea smooth cap
- Acetabularia crenulata serrated cap
The results of their studies showed that:
- The cap could be removed and it will still regenerate to type (can be done repeatedly)
- Hybrid cells can be constructed (stalk and holdfast) and the cap will regenerate. Cap will ultimately depend on nuclear type.
- Nuclear transplant experiments showed similar results
Therefore, the nucleus is the ultimate source of information and controls cellular activities. There are also cytoplasmic messengers which can influence cap type.
Location - Mitosis and Meiosis
The nucleus is the only part of the cell which accurately divides during cell division.
A precise mechanism is designed to separate the chromosomes accurately
Therefore, chromrosomes are the location of genetic material in eukaryotes:
- they dupliate precisely and divide accurately during somatic cell cycle
- in meiosis and fertilisation:
- chromosomes behave as expected (contributions from both parents)
- random mixing and crossing over (variability seen in offspring)
Chromosomes - Morgan et al
Morgan et al (1910, 1915)
- Experimented with Drosophila melanogaster
- Eye colouration and sex linkage
- Determination of linkage groups
- genes are linked
- sex linkage of certain loci
- determination of linkage groups
- number of linkage groups = number of chromosomes
Chromosomes - Phenotypic Changes
Chromosome changes associated with phenotypic changes:
- XX/XY sex chromosomes
- Down's syndrome - extra chromosome 21
- Klinefelter syndrome - XXY chromosome in males
- Turner syndrome - X_ chromosome in females
What is a chromosome made up of?
Miescher (1869) NUCLEIN
- extracted from human pus cell nuceli
- distinct from protein
- contains P
- resistant to proteases, i.e. DNA
- development of DNA specific dye
- DNA located in nucleus/chromosomes in both somatic and gametic tissue.
Chromosomes made up of...
ACIDIC DNA (nucelic acid) + BASIC PROTEIN (histones)
Todd and Levene (1920s)
- DNA composed of nucleotides, deoxyribose sugar, purine/pyrimidine base and P.
DNA is a very simple molecule, which proposed that it could not possible carry all the information necessary.
DNA concentration doubles over the cell cycle however, there are proteins contained within the chromosomes and their concentration will also double.
Genetic material would be argued over DNA or Protein for many years until experiments were performed to prove the DNA is the genetic material.
Hollaender and Emmons (1941)
Generated yeast mutants using UV irradiation.
They discovered that in order to generate the highest number of mutants, it is best to use UV light at 260nm.
The lmax DNA is 260nm
The lmax protein is 280nm
If the genetic material was a protein, you would expect more mutants to be generated under UV light at 280nm not 260nm.
Worked with Streptococus pneumoniae which is a capsulated bacterium that causes pneumonia in mice.
Two strains were experimented on:
S - capsulated, wild type form: pathogenic, smooth colonies when grown on agar.
R - non-capsulated, mutant form: non-pathogenic, rough colonies when grown on agar
Griffith added growning and heat treated (dead) pure cultures and mixtures to the mice. Obvserved the death rate and attempted to isolate viable bacteria from infected mice.
In R (non-pathogenic) - mice survived - no bacteria recovered
In S (pathogenic) - mice died - smooth bacteria recovered
In S heat treated (non-pathogenic) - mice survived - no bacteria recovered
In S heat treated + R living culture - mice died - smooth bacteria recovered TRANSFORMATION
Avery, Macleod and McCarty (1944)
Repeated the same exeperiments as Griffith but without the mice.
Mixed R: non pathogenic living culture WITH S: pathogenic, heated treated lysate OR semi purified fraction. They were placed directly onto the agar to rule out any influence of the mouse.
Sample used: crude bacterial lysate, DNA rich fraction (no treatment), DNA rich fraction + protease + RNase = TRANSFORMATION
Sample used: DNA rich fraction + DNase = NO TRANSFORMATION
Hershey and Chase (1952)
- structure and replication studies
- simple E.coli phage
- DNA in a protein coat
Prepared T2 phage that had either 32P DNA or 35S protein coat and used them to infect E.coli.
Their experiments showed that the 35S protein coat remained outside of the bacterial cells and the 32P DNA entered the cell and was found in the next generation of phage.
Watson and Crick (1953)
- The double helix
- Base pairing
- Complementary sequences
- Antiparallel strands
Not all genetic material is dsDNA...
- Using the same experimental design as Hershey and Chase, Sekiguchi et al (1960) showed that bacteriophage fX174 has a genome consisting of ssDNA
- Fraekel-Conrat et al and Gierer & Schramm (1956) showed that TMV19 has an RNA genome.
In eukaryotes, not all the DNA found in cells is in the nucleus..
- There is a DNA genome in chloroplasts and mitochondria. These look like prokaryotic genomes rather than the genome found in the nucleus.
DNA meets the criteria to be genetic material...
- Brenner, Crick & Jacob: Messenger Hypothesis which explains the link between DNA and the phenotype as seen in experiments on Acetabularia
- Crick: The Central Dogma of Molecular Biology - DNA (DNA replication takes place) ==> RNA (transcription) ==> Protein (translation). Exception of reroviruses and reverse transcriptase. RNA genome ==> DNA ==> RNA
- Not all RNA is designed to be translated