Powerpoint Templates

 Replication is semiconservative  Each strand is a template  Building starts at opposite ends (avoid friction of nucleotides)  Each new molecule is made of an old and a new strand.

 DNA helicase: breaks hydrogen bonds between nucleic acids  DNA polymerase: joins free nucleotides into new strands  DNA ligase: Joins new DNA strands to the old strand  Show animation now

Fig. 13-8a, p. 209 A Each DNA strand has two ends: one with a 5’ carbon, and one with a 3’ carbon. DNA polymerase can add nucleotides only at the 3’ carbon. In other words, DNA synthesis proceeds only in the 5’ to 3’ direction.

Fig. 13-8b, p. 209 The parent DNA double helix unwinds in this direction. Only one new DNA strand is assembled continuously. 5’ The other new DNA strand is assembled in many pieces. 3’ Gaps are sealed by DNA ligase. 5’ 3’ 5’ B Because DNA synthesis proceeds only in the 5’ to 3’ direction, only one of the two new DNA strands can be assembled in a single piece. The other new DNA strand forms in short segments, which are called Okazaki fragments after the two scientists who discovered them. DNA ligase joins the fragments into a continuous strand of DNA.

 DNA repair mechanisms › DNA polymerases proofread DNA sequences during DNA replication and repair damaged DNA  When proofreading and repair mechanisms fail, an error becomes a mutation – a permanent change in the DNA sequence

 DNA bases contains info to construct proteins.  Proteins determine our traits  Base pair  protein begins with transcription  DNA  mRNA  Protein

Nucleid Acid SugarNitrogen Bases StructureFunctionLocation DNADeoxyribos e Adenine Cytosine Guanine Thymine Double helix Contains genetic code nucleus RNARiboseAdenine Cytosine Guanine Uracil Single stranded mRNA: from nucleus to ribosome tRNA: aa to ribosome for assembly rRNA: structural componen e of ribsome with proteins Nucleus to cytoplasm cytoplasm Cytoplasm

 Only one region of one DNA strand is used as a template  RNA polymerase is used instead of DNA polymerase  The result of transcription is a single- stranded RNA

 RNA polymerase binds to the promoter region of DNA.  3’to 5’direction which assembles mRNA in the 5’to 3’direction.  As it moves RNA polymerase unwinds DNA a little bit at a time to read the base sequence.  While separated free RNA nucleotides are added to the exposed DNA bases.  Once the end of the gene is reached RNA polymerase releases both the DNA molecule and the newly formed mRNA.  End result: a copy of DNA sequence encoded in mRNA language.  SHOW ANIMATION NOW

 Many RNA polymerases can transcribe a gene at the same time

 In eukaryotes, RNA is modified before it leaves the nucleus as a mature mRNA  RNA copies of introns (DNA junk) are snipped out, leaving only exons (the good stuff).  Exons can be linked in order or rearranged and then spliced together. This allows for one gene to code for many diff. proteins. (alt. splicing)  After splicing, mRNA is modified with a guanine cap at the 5’end and a poly-A tail at the 3’end.

 mRNA Bases are read in groups of 3 (codons)  Each codon codes for a specific amino acid.  rRNA + proteins form the large and small subunits of a ribosomes. rRNA of the ribosome constructs peptide bond that links aa together.  tRNA has anticodons, and a specific aa.

 Genetic code › Consists of 64 mRNA codons (triplets) › Some amino acids can be coded by more than one codon  Some codons signal the start or end of a gene › AUG (methionine) is a start codon › UAA, UAG, and UGA are stop codons

 3 step process: initiation, elongation, termination  Coverts codons from mRNA into functioning proteins.

 In the cytoplasm.  Small ribosome subunit binds to the mRNA molecule  Anticodon of initiator tRNA binds with the start codon (AUG) of mRNA  Large subunit binds to the cluster and is now referred to as an initiatior complex.

 First aa is ALWAYS methionine.  The ribosome assembles a polypeptide chain as it moves along the mRNA  Results in the primary structure of proteins.

 Ribosomes will continue to add new aa to the chain until they reach a ‘stop’codon.  Polypeptide chain is released, the two ribosomal subunits separate and translation is complete

 Many ribosomes may simultaneously translate the same mRNA, forming polysomes