12/29/102 Functional segments of DNA Code for specific proteins Determined by amino acid sequence One gene-one protein hypothesis (not always true)

Molecular Basis of Inheritance

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• Functional segments of DNA• Code for specific proteins• Determined by amino acid

sequence• One gene-one protein hypothesis

(not always true)

GENES

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• 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

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• 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

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• 2 strands are complementary and antiparallel (run in opposite directions)

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• 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

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• 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.

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• 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

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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

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• 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

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Protein Synthesis

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• 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

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• 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

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• 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

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• Synthesis of polypeptide from mRNA

• Occurs at ribosomes

• tRNA contains anticodon which is complementary to codons in mRNA

Translation

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• 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

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• 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

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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

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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

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• 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

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• 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)

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• Trp operon- “turned on” unless too much of product is produced. Product can bind and change structure so that repression can bind

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• All cells contain the same genes. Some are turned on, some off. Different in each cell.

• Three different pathways:

GENE REGULATION IN EUKARYOTES

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• Chromatin structure- used to keep genes turned off, chromatin is more tightly wound in certain areas, it cannot be transcribed.

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• 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.

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• Translational- mRNA can be altered, so that it cannot be translated. The final polypeptide must fold correctly in order to be a functional protein.

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• 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

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• 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.

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• Many coding genes are expressed only part of the time, controlled by some mechanism