REPLICATIONIN BACTERIA

•Replication takes place at several locations simultaneously

•Each replication bubble represents 2 replication forks moving in opposite directions along the chromosome

•The bubbles continue to grow until they meet

DNA ploymerase• Proofreads whether hydrogen bonding is taking place

between the new nucleotides• When there is not bonding, means incorrect base and

DNA polymerase removes and replaces with correct base.

Termination

• Dismantling the replication machine• All the molcules and enzymes that work together the replicate DNA

• DNA Replication Animation 1• DNA Replication Animation 2

DNA sequencing• Identifying precise nucleotide sequence of a DNA

fragment• Human Genome Project

Human Genome

• Print-out showing the banded regions of human chromosome 14. Humans have 23 pairs of such chromosomes in their cells, making a total of 46 chromosomes. The chromosomes are a condensed form of the genetic material DNA (deoxyribonucleic acid). Each chromosome can be identified by its unique banding pattern. Each banded region can be identified with particular genes (short sections of DNA). Chromosome 14 in humans contains 886 genes, but also, like other chromosomes, large amounts of 'junk' DNA. The genes that have been identified on chromosome 14 include ones for Alzheimer's disease, Niemann-Pick disease, and thyroid disorders. This diagram was produced by Genoscope d'Evry (Genopole), France.

Protein Synthesis and Gene Expression

Section 18.2

Amino Acids• Proteins are made of sequences of amino acids• Only 20 different amino acids (aa) are found in

proteins• The DNA in a gene codes for these 20 aa’s by

combination of the 4 N-bases• What are these bases?

Central Dogma• Genetic information flowing from:• DNA RNA proteins gene expression

• There are two main stages of gene expression…

Gene Expression• 1. Transcription: Where genetic info is converted

from DNA messenger RNA (mRNA)• mRNA is translated by ribosomes into protein• It is a linear strand that carries information from the DNA in

the nucleus to protein synthesis machinery in the cytoplasm by passing through the pores of the nuclear envelope

Transcription• During transcription, the DNA sequence of a gene is

copied (transcribed) into the sequence of a single stranded mRNA molecule

Initiation

• RNA polymerase: main enzyme that catalyzes the synthesis of RNA• Promoter region is a sequence of nucleotides (in front of the gene) on

the DNA molecule that tells the RNA polymerase where to bind• RNA polymerase opens a section of the double helix DNA

Elongation

• Enzymes add nucleotides, in the 5’ 3’ direction, that are complementary to the template strand

• Only one strand is transcribed so no need for Okazaki fragments

Termination

• A specific codon in the template DNA serves as the signal to stop transcription (terminator codons)

• RNA polymerase detach from the DNA strand, new mRNA is released, DNA recoils into double helix

• Transcription Animation

Codons• During translation, the DNA code is read in groups of

3 nucleotide, called a codon• Each codon codes for a specific amino acid (aa) to be

placed in the growing polypeptide chain• Chain of 3 or more aa’s• Table 18.3 in text

The Genetic Code

Translation

• mRNA synthesizes a specific polypeptide chain• mRNA becomes an aa sequence through the translation of codons to

aa• This process requires a chemical translator and a set of cellular protein

synthesis equipment• These assemble proteins once mRNA reaches the cytoplasm

• Transfer RNA (tRNA) links each mRNA codon to it’s specific aa

• tRNA is made up of single strand RNA that folds into a characteristic lobe shape.

• One lobe contains the anticodon which is 3 nucleotides complementary to a codon on the mRNA

• The other end binds the corresponding aa

• Binding is done by specialized enzymes

• Ribosomes bring together the mRNA strand, the tRNA and the necessary enzymes

• They also contain ribosomal RNA (rRNA)

• A linear strand of RNA that remains associated with the ribosomes

Translation

• mRNA binds to active ribosome complex so that two codons are exposed

• The first tRNA molecule carrying the amino acid methionine (starter codon) pairs with the first exposed mRNA codon (AUG)

• Once tRNA and mRNA are in place, translation follows a cycle of three steps

Step 1

• Second tRNA molecule arrives at the adjacent codon to the first tRNA

Step 2

• The two aa’s become bonded through emzymes• The aa chain is transferred from the first tRNA to the second tRNA

Step 3

• The ribosome moves down one codon on the mRNA strand.• The first tRNA detaches from the mRNA and picks up another aa.• The second tRNA now hold the aa chain• A third tRNA arrives at the new codon next to the second tRNA and

the cycle repeats

• The translation continues until a stop codon is reached.

• The completed polypeptide chain is released and the ribosome assembly comes apart.

• http://www.mcgrawhill.ca/school/applets/abbio/ch18/proteinsynthesis_protei.swf

• Why DO Fireflies glow?