12/29/102 Functional segments of DNA Code for specific proteins Determined by amino acid sequence...
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Transcript of 12/29/102 Functional segments of DNA Code for specific proteins Determined by amino acid sequence...
Molecular Basis of Inheritance
212/29/10
• Functional segments of DNA• Code for specific proteins• Determined by amino acid
sequence• One gene-one protein hypothesis
(not always true)
GENES
312/29/10
• Discovered by Franklin/Watson/Crick• Composed of nucelotides– Pyrimidines- C/T– Purines- G/A– These always bond together (Chargaff’s Rule A-T C=G)
• Sugar and phosphate compose backbone
• Nitrogenous bases are variable and functional and compose genes
DNA STRUCTURE
412/29/10
• Covalent bond holds the phosphate to 5’C sugar on one end and 3’C sugar on the other
• DNA has a 5’(free P group) end and 3’ (free OH group) end
• N-bases attached to 1’C sugar and project into center, bonded by H bonds, AT (3 bonds), CG (2 bonds)
DNA SHAPE
612/29/10
• Replication is semi-conservative (1 old, 1 new strand)
• Helicase unwinds the DNA at many locations, replication fork starts in the middle of the strand and replication proceeds in both directions
DNA REPLICATION
712/29/10
• DNA polymerase adds complementary bases to the exposed strand, these bases are free and floating in cell
• DNA polymerase can only add bases in the 5’ to 3’ direction
• Leading strand goes in one directing, lagging strand has Okaski fragments that are later joined by ligase.
912/29/10
• DNA polymerase is very accurate• Many errors are corrected as
DNA strand is being formed• If error or physical damage
occurs, nucleases excise (cut out) the damaged portions and other enzymes then fill in the gaps
DNA REPAIR
1012/29/10
DNA VS RNA• DNA- nucleotides ATCG, double-
stranded, contains deoxyribose, large, more stable, contains genetic information
• RNA- nucleotides AUCG, single stranded, contains ribose, smaller, less stable, 3 types mRNA, tRNA, rRNA, directs protein formation
1112/29/10
• Messenger RNA– Copies genetic info from DNA– Carries message to ribosomes– Serves as template during translation
• Transfer RNA– Reads info in mRNA– Transfers proper amino acid to the
ribosome• Ribosomal RNA– Most of mass of ribosome– Stabilizes RNA template– Allows translation to proceed properly
TYPES OF RNA
1312/29/10
• Synthesis of RNA from DNA
• mRNA and tRNA transcribed in nucleus, rRNA in the nucleolus
• One strand of DNA is transcribed at a time
Transcription
1412/29/10
• RNA polymerase binds to specific DNA sequence in gene, called the promoter
• RNA polymerase causes DNA to unwind
• Another molecule of RNA polymerase brings in free RNA nucleotides and pairs with the exposed bases
• Uracil pairs with Adenine
• Many molecules can be transcribed at the same time
• Stops transcribing at termination signal
• DNA rewinds
• The pre-mRNA is processed, introns (non coding segments are removed), a cap and poly A tail is added
12/29/10
1612/29/10
• Codon- 3 mRNA bases, code for 1 amino acid
• 64 possible codons, but only 20 amino acids, therefore code is redundant
• Same for all living organisms• AUG- start codon- Met• UAG, UAA, UGA- stop codons
THE GENETIC CODE
1812/29/10
• Synthesis of polypeptide from mRNA
• Occurs at ribosomes
• tRNA contains anticodon which is complementary to codons in mRNA
Translation
1912/29/10
• Initiation- AUG becomes aligned in ribosome, initiator tRNA binds to ribosome and pairs with AUG, Met is bound
• Elongation- tRNA bonds with 2nd codon, peptide bond forms between Met and 2nd aa, Met detaches from tRNA and translation continues in 5’ to 3’ direction
• Termination- stop codon is reached, water molecule is added and polypeptide released from the ribosome, protein then folds into its proper shape
2112/29/10
• Spontaneous mutations can occur, mutagens can cause mutations, most are harmful
• Beneficial mutations lead to natural selection
• 2 main types: point and insertions or deletions
ERRORS/MUTATIONS IN GENES
2212/29/10
Point Mutations
• Change in 1 or few bases• Substitution- replacement of 1
pair of nucleotides, may or may not be harmful
• Missense- codon specifies wrong amino acid, may or may not be harmful
• Nonsense- codon changed to stop codon, nearly all lead to cell death
2312/29/10
Insertions/Deletion
• Insertion- addition of 1 or more nucleotides
• Deletion- loss of 1 or more nucleotides
• Both are harmful• Change the reading frame of
mRNA message• Frameshift- change all codons
after
2412/29/10
• Prokaryote Genomes- 1000’s of protein coding genes, most genes code for protein, only a small amount of non-coding DNA
• Eukaryote Genomes- 1000’s of protein coding genes, much of the genome does not code for proteins, many regulatory and repetitive regions
GENE REGULATION
2512/29/10
• Promoter- RNA polymerase attaches, to begin transcription
• Operator- small portion of DNA where an active repressor binds- when bound RNA polymerase cannot
• Structural Genes- one of many coding for amino acids that compose enzymes, transcribed as a unit
• Regulator Genes- located outside operon-controls whether or not an operon active or not.
GENE REGULATION IN PROKARYOTES (OPERON)
2612/29/10
• Trp operon- “turned on” unless too much of product is produced. Product can bind and change structure so that repression can bind
2812/29/10
• All cells contain the same genes. Some are turned on, some off. Different in each cell.
• Three different pathways:
GENE REGULATION IN EUKARYOTES
2912/29/10
• Chromatin structure- used to keep genes turned off, chromatin is more tightly wound in certain areas, it cannot be transcribed.
3012/29/10
• Transcriptional- the number of times a gene is copied can be controlled by silencers or enhancers. mRNA can leave the nucleus at different rates, more mRNA more protein product.
3112/29/10
• Translational- mRNA can be altered, so that it cannot be translated. The final polypeptide must fold correctly in order to be a functional protein.
3212/29/10
• Within the DNA molecule genes are located on both sides of DNA helix, with one gene often overlapping another gene. Much of DNA coding is not well understood and introns may help control gene expression
3312/29/10
• Transposons (jumping genes)- can alter gene expression. These genes can move around the genome and end up in the middle of a gene and prevent expression. Not well understood.