Chapter 10 packet: DNA and Protein Synthesis

Discovery of the structure of DNA DNA is in the shape of a double helix – discovered by Franklin & Wilkins through X-ray diffraction of DNA (a) Watson & Crick used above information to construct 1 st model of DNA (b)

Structure of DNA DNA is a made of nucleotides Nucleotides are composed of: Phosphate Sugar (deoxyribose) 1 of 4 nitrogen- containing bases - adenine (A), thymine (T), guanine (G), or cytosine (C)

Hydrogen bonds – holds 2 strands of nucleotides together Shape: double helix (Two strands twist around each other) Base pairing: A & T C & G (complementary base pairs)

DNA Replication Purpose: DNA makes an exact copy of itself prior to cell division; ensures that each new cell gets a complete copy of the DNA DNA Replication DNA Cell Division DNA

Steps: 1.An enzyme attaches to DNA and breaks H 2 bonds between bases – DNA chain unzips. 2.DNA polymerase (an enzyme) attaches to separated strand. 3.Complementary nucleotides bind to each side, forming 2 complete strands.

Overview of DNA replication

Ladder configuration and DNA replication

Each old strand (parent strand) of nucleotides serves as a template for each new strand (daughter strand). Proofreading and repair limits error rate to less than 1 per billion nucleotides. Website showing animation: rna2.swf Video clip from class:

DNA (Deoxyribonucleic acid) RNA (Ribonucleic acid) Sugar DeoxyriboseRibose Bases Adenine, thymine, guanine, cytosine Adenine, uracil, guanine, cytosine Strands Double-strandedSingle-stranded Helix YesNo Location NucleusNucleus, cytoplasm Types XXXXXXXXXMessenger, transfer, ribosomal

RNA vs. DNA

Messenger RNA (mRNA) - carries genetic information to the ribosomes Ribosomes - a part of the cell where proteins are made Ribosomal RNA (rRNA) - found in the ribosomes Transfer RNA (tRNA) - transfers amino acids to the ribosomes

Making Proteins A gene is a segment of DNA that specifies the amino acid sequence of a protein. DNA is found in the nucleus of a cell Proteins are made outside the nucleus at the ribosomes.

Overview of gene expression

Two processes are involved in the synthesis of proteins in the cell: Transcription – DNA is copied into mRNA, which will take a copy of the DNA code to the ribosome to direct the making of protein; occurs in nucleus Translation - the process of building proteins, the sequence of bases of mRNA is “translated” into a sequence of amino acids; occurs in ribosome

The Genetic Code DNA holds instructions to make a protein Instructions are copied into mRNA, which will be used to make a protein Codon - each three-letter unit of an mRNA molecule Each codon represents 1 amino acid There are 64 possible codons, and only 20 amino acids, so most amino acids have more than one codon

Messenger RNA codons

Transcription Purpose – Makes a copy of the DNA code that can leave the nucleus and travel to the ribosome to direct protein synthesis – mRNA Occurs in the nucleus Occurs at only 1 gene at a time Adenine in DNA pairs with uracil in RNA, not thymine Thymine in DNA pairs with adenine in RNA

Steps: 1.RNA polymerase attaches to DNA at the start of a gene 2.DNA unwinds and unzips 3.½ of DNA will serve as a template 4.Complementary bases are added along DNA 5.Once “stop” signal is reached, process ends, DNA closes back up, and mRNA is released Website showing animation: class.unl.edu/biochem/gp2/m_biology/animation/g ene/gene_a2.html

Transcription and mRNA synthesis

Translation Protein constructed during this process Occurs at the ribosomes Key players in translation: mRNA (messenger RNA) Made during transcription, has codons Travels from nucleus to ribosome Contains copy of DNA code to make protein tRNA (transfer RNA) rRNA (ribosomal RNA)

Transfer RNA (tRNA) tRNA molecules bring amino acids to the ribosomes Free-floating in the cytoplasm of the cell Anticodon – sequence of 3 bases on tRNA – it is this sequence that determines which amino acid each tRNA has

Complementary base pairing occurs between anticodons of tRNA and codons of mRNA – determines the sequence of amino acids to construct the polypeptide. If mRNA codon is AUG, tRNA anticodon would be UAC

Ribosomal RNA (rRNA) rRNA is made in the nucleolus (a cell structure found inside the nucleus) Makes ribosomes (location in cell where proteins will be made)

Steps: 1.Ribosome attaches to start codon on mRNA

2.tRNA brings amino acids to ribosome – codon on mRNA pairs with anticodon on tRNA

3. Amino acids are joined by peptide bonds

4. Stop codon is reached, mRNA is released, and protein is released class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3. html Website showing animation:

Gene Mutations Definition - a change in the sequence of bases within a gene Causes – Mutations can be spontaneous or caused by environmental influences called mutagens. Mutagens include radiation (X-rays, UV radiation), and organic chemicals (in cigarette smoke and pesticides).

Types of mutations – Insertion – One or more bases are inserted from a sequence of DNA Ex: AUGGGACCU  AUGGGGGACCU Can result in nonfunctional proteins Can result in no protein at all – stop codon where there shouldn’t be one

Deletion – One or more bases are deleted from a sequence of DNA Ex: AUGGGUACC  AUGGGACC Can result in nonfunctional proteins Can result in no protein at all – stop codon where there shouldn’t be one

Point mutations – One base is substituted for another May result in change of amino acid sequence Ex: UGC (Cysteine)  UGG (tryptophan) May not affect protein at all Ex: UAU  UAC both code for tyrosine May causes stop codon to occur early Ex: UAC  UAG (a stop codon)