AS Biology F212: DNA and RNA

DNA

The DNA needs to be stable and must be replicated accurately so daughter cells have the same genetic makeup.

When a cell divides into two,, the chromosomes have to replicate so hat each other the new cells still contains the original number of chromosome, e.g. 46 chromosomes in humans.

Because each chromosome consists of DNA, the DNA molecule has to REPLICATE. The parent cell produces a set of chromosomes identical to its own set. DNA replicates semi-conservatively.

Semi-conservative replication:

• Each parent strand acts as a template for a new strand
• Each new DNA double helix would then have one parent strand and one new strand 

Stages of DNA replication

• The whole  of the DNA molecule uncoils
• The DNA molecule unzips as the hydrogen bonds between the organic bases break
• The bases are now exposed
• In the nucleus individual DNA nucleotides are activated
• The bases of the free activated DNA nucleotides pair up with the complementary exposed bases on each original DNA strands
• The process of complementary base pairing ensures that C-G, A-T
• Covalent bonds form between the phosphate of one of the nucleotide and the sugar of the next to seal the sugar-phosphate backbone
• The whole process is controlled by the enzyme DNA POLYMERASE
• this continues all along the length of the DNA molecule until two DNA molecules are produced
• Both original strands are copied to give 1 old and 1 new strand.
• Two identical copies f the DNA are produced by the semi conservative method of replication

RNA

• RNA is found in three forms, (mRNA, rRNA, tRNA)

Difference between RNA and DNA

• (RNA
• DNA)

• RNA - found in the nucleus and cytoplasm
• DNA- found in the nucleus (small amounts in mitochondria and chloroplasts)
• Contains bases A, C, G, U
• Contains bases, A, C, G, T
• RNA molecule consists of a single strands
• DNA molecule consists of two strands running in opposite directions, twisted together to form a double helix
• Contains the pentose sugar ribose
• Contains the pentose sugar deoxyribose.

How DNA and RNA works together to produce a protein

The sequence of bases on DNA codes for particular polypeptide/protein molecules.

TRANSCRIPTION

• Part of the DNA uncoils and unzips 
• The hydrogen bonds break between the complementary base pairs
• The bases are exposed
• Only 1 of the DNA strand is used
• The particular sequence of bases for that gene form the template
• RNA nucleotide align next to the DNA template strand
• They join up individually using complementary base pairing, A with U, C with G
• A strand of mRNA is made when the backbone for the nucleotides are joined together
• DNA zips back up

AS Biology: Mitosis - Cell Cycle

CELL CYCLE

Interphase - Normal state of all cells. Chromosomes are not yet visible. During this stage cells carries out synthesis (growth) of cytoplasm and organelles such as mitochondria, enzymes and it increases in size. DNA replication also takes place in this stage so each chromosome consists of a pair of chromatids.

Prophase - Chromosomes become visible as chromatin fibres shorten and thicken by spiralisation. This condensation of chromosomes takes place. Once chromosomes are clearly visible they can be seen to consist of two chromatids. They have the same size and has an identical DNA base sequence.

Late prophase - 2 chromatids are twisted around one another and joined together by a centromere. The centrioles migrate to oppose ends of the cell and mictotubules develop to form fibres. The fibres make up a structure aka spindle. The spindle runs from pole to pole but is broadest at equator. Nucleolus disappears and finally the nuclear membrane breaks down.

Metaphase - Chromosomes line up at the equator of the spindle. Thy become attached to the spindle at their centromeres and line up across the equator

Anaphase - The centromeres divide into two and the spindle fibres pull the daughter centromeres apt. The separated chromatids are pulled along behind the centromeres. once separated the sister chromatids should now be called chromosomes and are now drawn to opposite poles.

Telophase - Chromosomes reach poles of the cell's spindle. The nuclear membrane reforms around each of the two groups of chromosomes and the nucleoli re-appears. The spindle fibres disintegrate and centrioles replicate  Prophase coiling sequence is reversed so that as the chromosomes uncoils and lengthen they cannot be seen clearly.

CYTOKINESIS - Telophase leads into cytokinesis (division of cytoplasm) so that daughter cells are formed. Cytokines differs in plant and animal cells. Animals undergo cleavage by constriction of the cytoplasm and furrowing the plasma membrane in plants a cell plate forms across the equator.

(Plants) - In animals most cells are capables f cytokinesis  whereas in plants only special cells aka meristems can divide in this way. They are found at the root and shoot tips. Meristem tissues are responsible for the growth of the whole organism. Plants cells that do not have centrioles the tubulin protein threads are made in the cytoplasm

(Animals) - In animals cells cytokinesis starts from outside working inwards to the cell membrane but in pants cells it starts with the formation of the cell plate where the spindle equator was. New cell mebrane and new cell was material is laid down along this cell plate.

Yeast cells undergo cytokinesis by producing a small bud that nips of the cell, in a process called budding.

ROLL OF MITOSIS

• Asexual reproduction e.g. propagation in plants
• Growth - of multicelluar organsims by producing extra cells. 
• Replacement - e.g. RBC and skin cells are replaced by new ones.
• Repair - damaged cells that needs to be replaced by identical new ones.

Features of mitosis: chromosome number is maintained there is no change in genetic material.