(deoxyribonucleic acid) components of dna...dna (deoxyribonucleic acid) • genetic material of both...
TRANSCRIPT
Life Goes On
Dr. Kwok Cheong CHUNG Department of Biology
The Chinese University of Hong Kong
6 th International Junior Science Olympiad (IJSO)
Molecular Genetics
1
Contents
• DNA structure • DNA replication • Genetic Code • Transcription • Translation
2
Notes to Teachers • Teaching Objectives
– To let students know the basic structure of DNA molecules (0.5 hr)
– To let students know how DNA is replicated (0.5 hr) – To let students know the genetic code (0.5 hr) – To let students know how genetic information in the
DNA molecule is transferred to RNA molecules through the process of transcription (1 hr)
– To let students know how the genetic information in RNA molecules is used as a template for protein synthesis through the process of translation (1 hr)
• Time allocation: 3.5 hours
3
Learning Outcomes
• know the basic structural components of DNA molecules
• know how DNA is replicated • how genetic information in the DNA molecule is transferred to RNA molecules through the process of transcription
• know how the genetic information in RNA molecules is used as a template for protein synthesis through the process of translation
After studying this topic students will be able to:
4
DNA (deoxyribonucleic acid) • Genetic material of both eukaryotes and prokaryotes • Many viruses have DNA, but some have RNA genomes instead
• Genes are specific sequences of nucleotides that pass traits from parents to offspring
• Genetic material in cells is organized into chromosomes
• Prokaryotes generally have one circular chromosome • Eukaryotes generally have:
a)Linear chromosomes in nucleus, with different species having different numbers of chromosomes
b)DNA in organelles (e.g., mitochondria and chloroplasts) that is usually a circular molecule 5
Components of DNA • Polymers of nucleotides • Nitrogenous base + deoxyribose + phosphoric acid
• Four bases in DNA: A (adenine), G (guanine), C (cytosine) and T (thymine)
• In RNA – Ribose replace deoxyribose – Uracil replace thymine
Phosphate Group
Deoxyribose sugar
Base
Adenine Cytosine Guanine Thymine
OH group in RNA
6
Components of DNA • Nitrogenous bases
–Purines – double ring –Pymiridines – single ring
• Deoxyadenosinemonophosphate dAMP
• Deoxyguanosinemonophosphate dGMP
• Deoxycytidinemonophosphate dCMP
• Deoxythymidinemonophosphate dTMP
Purine Pyrimidine
Adenine
dAMP
A
dAMP dATP dADP
Cytosine C
dCMP
Thymine T
dCTP
Guanine G
dGMP
7
DNA Structure • Two nucleotide chains running antiparallelly and joined by Hbonds between base pairs “double helix”
8
DNA Directionality • Determined by the anti parallel arrangement of the DNA strand
• Each end is labeled as either the 5’ or 3’ end according to the terminal carbon’s positional number in the deoxyribose ring
• The 5’ carbon has a phosphate attached whereas the 3’ carbon has a hydroxyl group attached
9
The Genetic Code
• Base sequence determines genetic information
• Triplet code three bases code for an amino acid
• 3nucleotides = a Codon • In each position 4 bases possible 4x4x4 = 64 codons
• 3 stop codons • Remaining 61 codons code for 20 amino acids
• Codon & anticodon 10
DNA Form Facilitates Function • Consists of two complementary strands of nucleotides that can serve as templates for the ordering of nucleotides in the creation of new copies of DNA
• How the parent strands of DNA create new copies of DNA? • Semiconservative model of DNA replication each daughter strand will receive one of the original strands
11
DNA Replication • Origins of Replication are the sites of new DNA synthesis
• The origin of replication opens up the DNA strand and creates two replication forks where new DNA is synthesized in a specific direction
• In bacterial chromosomes only one origin of replication
• In eukaryotic cells thousands of origins of replication
Replication bubbles
12
DNA Synthesis • New DNA is only made in the
5’ to 3’ direction new nucleotides are only added to the 3’ end of the strand
3’ carbon
3’
5’
5’ 13
Replication Forks
• Split section of the parent strands that allows the enzymes access to the DNA template for replication
14
Addition of Nucleotides • The addition of a nucleotide to the emerging strand of DNA is
accomplished by the formation of a phosphodiester bond between the 5’end of a nucleoside triphosphate and the hydroxyl group of the 3’ end of the adjacent nucleotide in the chain
• The NTP loses a twophosphate molecule called a pyrophosphate
• Hydrolysis of the pyrophosphate releases the energy to drive the polymerization reaction
Exergonic reaction
15
Replication Enzymes • DNA polymerases: polymerize DNA molecules. The DNA polymerases use the parent strand as a template to string together nucleotides in a complementary or daughter strand
• Helicases: Are enzymes that unwind the DNA double helix at the replication fork, so that the two strands are separated. Single stranded binding proteins then line along the two strands to keep them apart and accessible to the replication enzymes
• Primases: DNA polymerases can not start a DNA chain from scratch. They need a primer. The primer is a short stretch of RNA that is used to jump start the DNA synthesis process. Primases are responsible for creating this RNA segment from scratch at an origin of replication. Later another DNA polymerase will come along and replace this RNA segment with the DNA version 16
Replication Enzymes • Ligases: These enzymes are responsible for linking together the Okazaki fragments in the lagging strand after the DNA polymerases have replaced the RNA primers with DNA
• Nucleases: A DNA cutting enzyme. This enzyme is used in repair of DNA damage. The nuclease cuts out the damaged section so that polymerases can fill it in with the proper nucleotides
• Telomerases: These enzymes are responsible for elongating telomeres. They are special enzymes that contain a short RNA segment to use as its own template to elongate the telomere
17
Leading Strand Elongation • The leading strand can be continuously elongated toward the replication fork because it is continuously adding nucleotides to the 3’ end of the strand
• The parent/template strand begins with the 3’ end and the complementary strand or daughter strand starts with the 5’end
18
Lagging Strand Elongation • DNA polymerase works in the opposite direction away from the replication fork
• creates a short segment of DNA and as the replication bubble grows another short strand can be made Okazaki fragments
• Require RNA primer be made by a primase
• DNA ligase “ligates” the sugarphosphate backbone of the daughter strand creating one long complementary DNA strand
singlestrand binding protein
19
Ligase Joins Okazaki Fragments
20
When it all goes wrong: DNA Repair • The DNA polymerase self corrects by proofreading the newly
created daughter strand and replacing incorrectly paired bases • If a base pair error is missed or occurs as a result of some sort of
nucleotide damage a mismatch repair occurs. Special enzymes are employed to remove the incorrect bases and replace them with the correct ones
• Colon cancer has been linked to a hereditary defect in one of these special enzymes that does not correct the mismatch and allows cancer causing errors to accumulate in the DNA
• Damage that needs repair can also occur to existing DNA. Chemical and physical damage (toxins, xrays, UV radiation, etc) can cause changes in the DNA. Spontaneous chemical changes to the bases also occur in the cell under normal conditions. This typically calls for nucleotide excision repair, where a nuclease (an enzyme that cuts out sections of DNA) cuts out the damaged section and then a DNA polymerase replaces the cut out section, then the sugarphosphate backbone is repaired by a ligase. 21
Telomeres • The fact that the DNA polymerase can only add new nucleotides to the 3’ end of existing nucleotides is a limitation that can cause a potential problem for organisms with linear DNA. The DNA polymerase can not finish the lagging strand. This would result in the deletion of genes.
• To combat this problem eukaryotic chromosomes have telomeres at the ends of their DNA strands
• Telomeres are sequences of noncoding nucleotides that will not create any defects in the organism if they are deleted
• In order to keep the telomeres from becoming increasingly shorter over time some cells have enzymes called telomerases that have their own template that allows them to elongate DNA at the 3’ end of the strand, then the polymerases extend the 5’ end in the usual fashion. 22
How Can the Code be Decoded? • Two steps in the production of a protein
• Transcription (nucleus) – DNA base sequence copied into messenger RNA (mRNA) sequence
• Translation (cytosol) – mRNA sequence read for synthesis of protein at the ribosome
• DNAà RNAà Protein 23
Central Dogma of Molecular Biology
DNA
mRNA
Protein
Transcription
Translation
Transcription
Translation
24
Central Dogma of Molecular Biology
DNA
mRNA
Protein
Transcription
Translation
Complementary base pairing ensures the correct transfer of information between DNA in replication and DNA to RNA in transcription blueprints for the cellular machine
In analogous manner, same applies to correct transfer of information between mRNA and polypeptides
Proteins (and RNA, sometimes) perform biological work, analogous to the machine itself 25
Transcription • Initiation RNA polymerase attach to the gene at the promoter – Genes are read 3’ to 5’ – mRNA is made 5’ to 3’
• New nucleotides added elongation
• Stop at the stop codon termination
• Completed RNA transcript detached
26
Promoter • Lies upstream of the DNA sequenced
• Allows RNA polymerase to attach to DNA strand
• Signals to the RNA polymerase which side of the DNA is to be transcribed
27
Transcription Cont’d • After initiation RNA polymerase will open about 10 to 20 nucleotides at a time
• RNA polymerase will continue past the terminator and cut the mRNA off
• Many RNA polymerase complexes will be transcribing the gene at one time
• Direction of transcription 5’→3’
28
mRNA Processing • A guanine cap placed onto the mRNA at the 5’ end
– Helps protect the mRNA sequence – Attaches the unit to the ribosome
• A poly A tail is attached to 3’ end – Helping in attachment and stopping hydrolytic processes
– Aids in getting the sequence out of the nucleus
29
mRNA Splicing (Eukaryotes) • Splisosomes
– Compose of snRNP’s (small nuclear ribonucleoprotein particles, made out of snRNA) and proteins
– Small nuclear RNA (snRNA) is a class of small RNA molecules in the nucleus of eukaryotic cells. Involved in a variety of important processes such as RNA splicing
– Introns will be taken out from the premRNA – Exons will be bound together to form mRNA
30
Translation • Reading of the codons on the mRNA strand and the assembling of amino acid residues according to the sequence
• mRNA processed within the nucleus, template for the sequence of amino acids
• tRNA transfers amino acids from the cytoplasm to the ribosome
• Ribosome adds amino acids together from the tRNA and in the sequence of the mRNA
31
tRNA • Synthesized in nucleus • ~ 80 nucleotides, Tshaped
– Pick up designated amino acids in the cytosol
– Deposit the amino acid at the ribosome
– Return to the cytosol to pick up another amino acid
• Base sequence of anticodon pairs with the codon on the mRNA strand
• Different tRNAs for different AAs
32
Ribosome
• Composed of RNA and proteins • Ribosomal RNA (rRNA) the largest type of RNA
• Two subunits • Binding site for mRNA and 3 binding sites for tRNA molecules
• P site holds the tRNA carrying the growing polypeptide chain
• A site carries the tRNA with the next amino acid
• Discharged tRNAs leave the ribosome at the E site
33
Translation • Initiation, Elongation, Termination • Initiation
– Initiator tRNA (Met) + small ribosomal subunit + large ribosomal subunit
– Energy needed to bind the tRNA to the P site
34
Elongation • Codon Recognition (GTP needed) – The anti codon HBonds with the codon – Requires energy (A site)
• Peptide Bond Formation – Peptide is formed with AA in P to A site
– Catalyzed by the ribosome
• Translocation (GTP needed) – tRNA in the P site and A site moved to the E site and P site respectively
– The tRNA in the E site will detach and a new codon is open
35
Termination • One of three stop codons attaches to the A site • Polypeptide released
36
Summary
37
The Central Dogma • The central dogma describes the flow of hereditary information from DNA to RNA to protein
• Genetic information is encoded by the sequence of the four DNA bases (A, C, G, and T) • RNA also has four bases (A, C, G, U) • Protein has 20 amino acids
• Transcription is DNA to premRNA, translation is mRNA to polypeptide
38
2 3 4 5 DNAà premRNAà mRNAà Immature à Final
protein protein 1
DNA
1. Replication 2. Transcription 3. Posttranscriptional processing 4. Translation 5. Posttranslational processing
DNA
Protein
39
References for Further Studies • Introduction to DNA Structure
– http://www.blc.arizona.edu/Molecular_Graphics/DNA_St ructure/DNA_Tutorial.HTML
– http://www.youtube.com/watch?v=qy8dk5iS1f0 – http://en.wikipedia.org/wiki/DNA
• DNA replication – http://en.wikipedia.org/wiki/DNA_replication – http://www.youtube.com/watch?v=teV62zrm2P0
• Central dogma of molecular biology – http://en.wikipedia.org/wiki/Central_dogma_of_molecula r_biology
– http://www.accessexcellence.org/RC/VL/GG/central.php – http://www.youtube.com/watch?v=ygpqVr7_xs 40