Nucleic Acids & Protein Synthesis Chapter 3

Structure of DNA Chromosomes are composed of DNA - DeoxyriboNucleic Acid DNA is double stranded Each strand is made of nucleotides Nucleotides are made of deoxyribose sugar, a phosphate group and a base Bases: adenine (A), guanine (G), cytosine (C), thymine (T) Strong chemical bonds between phosphates & sugars join nucleotides - sugar-phosphate backbone Hydrogen bonds join bases from one strand to a complimentary base on the other strand : A-T, G-C This forms a double helix (spiral ladder)

Structure of RNA Also consists of nucleotides, BUT 1) Is single stranded - DNA is double stranded 2) Has the base uracil (U) - instead of thymine (T) 3) Has a ribose sugar - instead of deoxyribose

DNA base sequence Order & quantity of DNA bases establishes differences between living things Protein structure/function is dependant upon amino acid sequence Amino acid sequence depends on DNA base sequence - the genetic code Every 3 bases of DNA will correspond to a specific amino acid

Protein Synthesis Proteins are made in the cytoplasm A messenger is required to pass information from the DNA to the cytoplasm This is messenger RNA (mRNA) It is formed (transcribed) from free RNA nucleotides in the nucleus This process of transcription involves 8 stages

Transcription of DNA into mRNA 1) DNA double helix unwinds 2) H bonds between bases start to break 3) Free RNA nucleotides pair with their complimentary DNA nucleotides 4) H bonds form between RNA and DNA 5) An enzyme (RNA polymerase) helps to join RNA nucleotides 6) mRNA separates from DNA template 7) mRNA leaves the nucleus through a pore in the nuclear membrane 8) H bonds reform to reunite the DNA chains – double helix reforms

Translation (RNA to protein) mRNA leaves the nucleus and moves to the cytoplasm mRNA becomes attached to a ribosome The mRNA is then translated to form a protein - every 3 bases forms a CODON - these correspond to an amino acid Transfer RNA (tRNA) is found in the cytoplasm It brings amino acids to the ribosome tRNA has triplets of bases - ANTICODONS Anticodons match up with complimentary CODONS on the mRNA Each amino acid arrives in sequence Amino acids then joined by peptide bonds

Translation Video Clip

Cell Structure Nucleolus : Within the nucleus – made of DNA, RNA & protein (help to make ribosomes) Ribosome: found in cytoplasm – translation of mRNA into protein Free ribosomes – protein stays inside cell Attached to Endoplasmic Reticulum (ER) – proteins out of cell

Cell Structure Endoplasmic Reticulum (ER): Rough ER: Encrusted with ribosomes - pathway for transport of materials - proteins synthesised on ribosome, injected into ER, coiled & folded - passed on for exiting the cell Smooth ER: no ribosomes - Various roles – site of many reactions

Cell Structure Golgi Apparatus: Picks up proteins from rough ER (in vesicles) Proteins processed, & passed through (in vesicles) Vesicles with finished product are pinched off from the end – discharged out of cell This is called exocytosis Golgi Apparatus

Cell Structure Lysosomes - vesicles containing digestive enzymes - cell repair, cell defence, food digestion