Nucleic Acids & Cell Division

• individual nucleotides are made of 3 components combined by condensation reactions
  • phosphate group= consistant structure in all nucleotides
  • pentose sugar
  • organic base containing nitrogen
    • pyrimidine bases= thymine & cytosine
    • purine bases= adenine & guanine
• DNA is a double stranded polymer of nucleotides or polynucleotide
• each polynucleotide is made up of millions of nucleotide units
• in the form of a double helix maintained by hydrogen bonds between bases
• pentose sugar is always deoxyribose
• four organic bases in complementary pairs-
  • cytosine & guanine / adenine & thymine

• DNA  is a very stable molecule and can pass through generations without change
• very large molecule carrying large amount of genetic info
• two strands separate easily as hydrogen bonds are weak 
• base pairs held within deoxyribose-phosphate backbones & is therefore protected

• single stranded polynucleotide with pentose sugar ribose
• RNA has organic bases cytosine & guanine, adenine & uracil
• mRNA- long single stranded molecule formed into a helix manufactured in nucleus carryig genetic code from DNA to ribosomes in cytoplasm
  • rRNA- found in cytoplasm as a large complex unit of double and single helices e.g ribosomes have rRNA bound to protein. site of translation of the genetic code
    • tRNA- small single strand molecule forming a clover shape with c-c-a sequence at one end for amino acid to attach and anticodon at the other end. transports amino acids to rbosome for protein synthesis

DNA   RNA 

• sugar is deoxyribose
  • sugar is ribose
• bases are cytosine & guanine; adenine & thymine
  • bases are cytosine & guanine; adenine & uracil
• double helix
  • single helix

• made from DNA, protein and a small amount of RNA 
• DNA occurs as a single strand of a double helix running the lenth of the chromosome
• each DNA molecule has manyy sections of genes
• before cell division each DNA molecule makes a copy of itself, forming two identical chromatid threads that lie parallel to eachother and are joined at the centromere

• chromsome numbers vary- in humans it is 46 chromosomes in 23 homologous pairs
• this is the diploid number
• half of the diploid number is the haploid number found in gametes - 23 chromosomes

• two daughter cells genetically identical to the parent cell
• dividing cells undergo a cell cycle of 4 stages plus a resting stage between each division

interphase

• longest part of the cell cycle
• newly formed cell increases in size and produces organelles
• quantity of DNA is doubled
• before the next division the chromosomes replicate to form two chromatids joined at the centromere
• ATP is needed
• chromosomes are not visible as chromatin is dispersed throughout the nucleus

• longest stage in mitosis
• chromosomes condense to become visible as long threads- known as chromatids
• centrioles divide to reach the poles, synthesise protein microtubles to make spindles
• nuclear membrane disintergrates and nucleolus disappears
• chromatid pairs are free in the cytoplasm

metaphase

• chromosomes arrange themselves at equator and attach to spindle fibres by centromere
• contractions of the spindles pull chromatids slightly apart

anaphase

• rapid stage
• centromere splits
• spindle fibres contractions pull the separated chromatids to the poles centromere first

telophase

• chromosomes have reached the poles, uncoil and lengthen
• spindle breaks down, nucleoli disappear and nuclear membrane reforms
• cytokinesis constructs the new cell membrane from outside to in (animals)
• cell plate laid down from inside to out (plant)

• two cells with the same number of chromosomes as parent
• genetic stability through making genetic identicals to parent
• growth of an organism
• repair of tissues/ replacement of dead cells- eg root tip/ skin cells
• asexual reproduction
• yeast bacteria insects 
• flowering plants - bulbs runners tubers
• large numbers of offspring in a short amount of time
• no variation

• 2 gametes fuse to form a zygote
• need haploid numbers in these gametes to keep chromosome number stable
• meiosis 1- chromosome nuber is halved
• meiosis 2- mitosis of the hapliods for identical daughters
• result of 4 daughter nuclei with each half the number of chromosomes of the parent
• prophase 1- homologous chromosomes wrap around eachother and partially repel but remain joined at certain points called chiasmata 
• chromosomes may break and recombine- this is crossing over and a source of genetic variation
• during first stage of metaphase chromosomes arrange themselves randomly at equator
• random distributions produces new genetic combinations

Mitosis    Meiosis

• one division 2 daughter cells
  • two divisions four daughter cells
• number of chromosomes is constant
  • number of chromosomes halved
• homologous chromosomes not in pairs
  • homologous chromosomes pair up
• no crossing over
  • chiasmata form and crossing over occurs
• daughter cells genetically identical
  • daughter cells genetically different

• for a species to survive in a constantly changing enviroment and to colonise new environments variation is essential
• sexual reproduction mixes the genotypes of two parents
• independant assortment means gametes with different combinations of chromosomes
• during crossing over chromosome parts are exchanged producing new combinations and separating linked genes