Made up from :
- A phosphate group
- A pentose sugar
- A nitrogenous base, either purine (G, A) or pyriminidine (C, T)
- Double stranded polymer
- Each polynucleotide contaisn thousands of units
- Double helix is maintained by hydrogen bonding
- Pentose sugar is always deoxyribose.
- The four organic bases are Cytosine, Guanine, Thymine and Adenine.
- Each strand is linked to the other by these bases.
- Thymine is complementary to Adenine, Guanines to Cytosine.
Why DNA is good for its function:
- Very stable: can be passed down generations wthout change.
- Very long: can contained a lot of information needed for the coding of proteins.
- Sugar Phosphate backbone protects DNA from external forces.
- Hydrogen bonds easily broken for transicription.
- Single stranded
- Pentose sugar is Ribose
- Adenine, Guanine, Cytosine and Uracil (replacing Thymine)
Three types of RNA:
- mRNA- Messenger RNA. This is a copy of DNA that moved out the nucleus, and is processed in a ribosome to make a polypeptide.
- rRNA- Ribosomal RNA. Large molecule made from double and single helices. This, along with proteins, makes up ribosomes.
- tRNA- Transfer RNA. Small single stranded molecule that forms a clover-leaf shape. It transports amino acids to ribosomes.
Chromosones and Interphase
DNA runs the length of a chromosome, that is split into many ections called genes.Chromosones are homlogous, and contain a set from the mother and one from the father. Humans have 23 pairs of chromosones in diploid cells (46 chromosones overall). Gametes have 23 chromosones overall.
Shortly before mitosis begins, the chromosones duplicate, forming chromatids, joined by the centromere.
This is a 'resting' period for the cell before mitosis. Three things occur:
- Cell size increases (growth)
- DNA is replicated
- Produces new organelles to replace lost ones.
The nuclear envelope disintegrates, and chromatin condenses into sister chromatids. Centrioles move to each pole of the cell.
Sister chromatids line up with the centronome on the equator of the cell. Spindle fibres are proudiced by the centrioles, and attach to each sode of the centronome.
The spindle fibres contract and shorten, pulling the chromatids to each pole of the cell.
Two new nuclear membranes begin to form around chromosones, which also begin to become chromatin again.
FINISHED BY CYTOKINESIS - where the whole cell splits.
Significance of Mitosis
- Produces diploid cells thate are genetically identical to the parents. This craetes stability within the organism.
- Used for growth and repair.
- Asexual reproduction is using in many organisms, as it quick and effective at producing offspring.
- Two rounds of cytokinesis to produce 4 daugher cells.
- Daughter cells are haploid gametes.
- Daughter cells are no identical.
Genetic Variation is vital fo survival and adaptation in changing environments.
- Crossing over: During Metaphase the chromatids wrap so tightly together, that certain genes can swap at places called chiasmata. This means the chromosones are genitically changed.
- Independent assortment: The spindle fibres could pull any chromosone towards it pole, therefore the chromosones ineach daughter cell is completely random, resulting in genetic variation.
- The two parents can be random, so when they fuse this gives genetic vaiation.
Mitosis v Meiosis
- Mitosis produces genetically identical daughter cells; Meiosis produces genetically different.
- Mitosis has one round of cytokinesis to make two daughter cells. Meiosis has two rounds to produce 4.
- Mitosis produces diploid cells; Meiosis produces haploid cells.
- Homologous chromosones do not associate in pairs in Mitosis, but they do in meiosis.
- HOW MANY DAUGTERS
- HOMOLOGOUS PAIRS