Chapters 13: DNA Structure & Function Ch.14 From DNA to Protein OCC BIO-114 By Dave Werner

Chapters 13 & 14 Overview DNA – Replication RNA - Transcription Protein Synthesis - Translation

DNA Ultimately the genetic source of this amazing diversity is deoxyribonucleic acid. The primary function of DNA in organisms is to store and transmit the genetic information that tells cells which proteins to make and when to make them. Proteins in turn form the structural units of cells and help control chemical processes within cells.

Structure of DNA (fig.13-5) DNA is an organic compound made up of 2 long chains of nucleotides DNA is made up of 1. sugar molecule – deoxyribose 2. phosphate group – consists of phosphorus atom surrounded by oxygen atoms. 3. nitrogen-containing base

Structure of DNA – Nitrogen Bases (fig.13-4) The four nitrogen-containing bases found in DNA nucleotides are: Adenine Guanine Cytosine Thymine A G C T

Structure of DNA (fig.13.5)

Structure of DNA In 1953, James Watson and Francis Crick suggested a model for the structure of DNA. This shape is known as the double helix

Rosalind Franklin’s Work Was an expert in x-ray crystallography Used this technique to examine DNA fibers Concluded that DNA was some sort of helix

Structure of DNA Individual nucleotides are connected by covalent bonds between the deoxyribose sugar and the phosphate molecules The alternating deoxyribose sugar and phosphate molecules form a “backbone” to which the nitrogen-containing bases attach.

Structure of DNA By facing toward the center, the bases on one chain of DNA face the bases of the other chain of DNA Form hydrogen bonds.

Complementary Base Pairing Cytosine pairs with Guanine C-G Adenine pairs with Thymine A-T These pairs are known as complementary base pairs.

Nucleotide Bases ADENINE (A) GUANINE (G) phosphate group deoxyribose THYMINE (T) CYTOSINE (C)

Replication of DNA (fig.13-6) Replication is the process of copying DNA The 2 nucleotide chains separate by unwinding, and each chain serves as a template for a new nucleotide chain. Replication fork is the point at which the 2 chains separate

DNA Replication Each parent strand remains intact Every DNA molecule is half “old” and half “new” new old old new

Replication of DNA The chains are separated by enzymes called helicases DNA polymerases are enzymes that bind to the separated chains of DNA As DNA polymerases move along the separated chains, new chains of DNA are assembled using nucleotides in the surrounding medium that are complementary to the existing DNA chains

Replication of DNA When replication is completed, 2 new exact copies of the original DNA molecule are produced and the cell is ready to undergo cell division What phase of the cell cycle does this happen?

Accuracy and Repair The # of errors and mutations in DNA replication is reduced as enzymes proofread DNA and repair errors. 1 error in 10,000 goes from 1 in 1 billion b/c of proofreading & repair Error = mutation Mutations can occur from chemicals, UV, and other agents.

Ch.14 From DNA to Protein Recall that the nucleotides in DNA molecules are grouped into genes that contain the information needed to make specific proteins. In eukaryotes, the genes detecting protein production are in the nucleus, and the enzymes and amino acid building blocks for protein production are in the cytosol. Nucleic acid called ribonucleic acid is responsible for the movement of genetic information from the DNA in the nucleus to the site of protein synthesis in the cytosol

Structure of RNA RNA structure is similar to that of DNA, except: The sugar in RNA is ribose, not deoxyribose Uracil, a nitrogen containing pyrimidine base, replaces thymine in RNA

DNA vs. RNA DNA Bases RNA Bases Uricil Base (U) Thymine Base (T)

3 Types of RNA 1. Messenger RNA (mRNA) consists of RNA nucleotides in the form of a single uncoiled chain. mRNA carries genetic information from the DNA in the nucleus to the cytosol of a eukaryotic cell.

3 Types of RNA 2. Transfer RNA (tRNA) consists of a single chain of about 80 RNA nucleotides folded into a hairpin shape that binds to specific amino acids. There are about 45 varieties of tRNA

3 Types of RNA 3. Ribosomal RNA (rRNA) – the most abundant form of RNA. rRNA consists of RNA nucleotides in a globular form. Joined by proteins, rRNA makes up the ribosomes where proteins are made

Transcription Transcription is the process by which genetic information is copied from DNA to RNA. RNA polymerase, the primary transcription enzyme, synthesizes RNA copies of specific sequences of DNA

Gene Transcription DNA to be transcribed DNA winds up again transcribed unwinds transcribed DNA winds up again mRNA transcript RNA polymerase

Transcription RNA polymerase initiates RNA transcription by binding to specific regions of DNA called promoters The promoter marks the beginning of the DNA chain that will be transcribed

Transcription When RNA polymerase binds to a promoter, the DNA molecule in that region separates. Only one of the separated DNA chains called the template is used in transcription

Transcription RNA polymerase attaches to the first DNA nucleotide of the template chain Then it begins adding complementary RNA nucleotides to the newly forming RNA molecule The base pairing rules are identical to those in DNA replication, except that uracil pairs with adenine

Transcription Transcription continues one nucleotide at a time until the RNA polymerase reaches a DNA region called the termination signal. Termination signal is a specific sequence of nucleotides that marks the end of a gene.

Transcription At the termination signal, RNA polymerase releases both the DNA molecule and the newly formed RNA molecule All three types of RNA are transcribed in this process.

Adding Nucleotides 5’ growing RNA transcript 3’ 5’ 3’ direction of transcription

Transcript Modification Fig. 14.4 unit of transcription in a DNA strand 3’ exon intron exon intron exon 5’ transcription into pre-mRNA poly-A tail cap 5’ 3’ snipped out snipped out 5’ 3’ mature mRNA transcript

Transcription The products of transcription are the different types of RNA molecules, including mRNA, tRNA, and rRNA. Following transcription, mRNA moves through the pores of the nuclear membrane into the cytosol of the cell where it will direct the synthesis or translation of the protein.

Protein Synthesis The production of proteins is also called protein synthesis. The amount and kind of proteins that are produced in a cell determine the structure and function of the cell. In this way, proteins carry out the genetic instructions encoded in an organism’s DNA.

Protein Structure and Composition Proteins are made up of one or more polypeptides, each which consists of a specific sequence of amino acids linked together by peptide bonds. The sequence of the amino acids determines how the polypeptides will twist and fold into the three-dimensional structure of the protein

The Genetic Code During protein synthesis, the sequence of nucleotides in an mRNA transcript is translated into a sequence of amino acids. Genetic Code is the correlation between a nucleotide sequence and amino acid sequence This genetic code is used by most organisms to translate mRNA transcripts into proteins

Genetic Code Set of 64 base triplets Codons 61 specify amino acids Nucleotide bases read in blocks of three 61 specify amino acids 3 stop translation

Code Is Redundant Twenty kinds of amino acids are specified by 61 codons Most amino acids can be specified by more than one codon Six codons specify leucine UUA, UUG, CUU, CUC, CUA, CUG

Redundant? (Genetic Code Secret Decoder Ring)

The Genetic Code Codons are 3 mRNA nucleotides which code for a specific amino acid. The Start Codon (AUG), which also codes for the amino acid methionine, engages a ribosome to start translating an mRNA molecule Stop Codons (UAA, UAG, UGA) cause the ribosome to stop translating an mRNA

Translation Translation is the process of assembling polypeptides from information encoded in mRNA This process starts when mRNA leaves the nucleus through pores in the nuclear membrane. The mRNA then migrates to a ribosome in to cytosol

Translation Amino acids floating freely in the cytosol are transported to the ribosomes by tRNA molecules. tRNA anticodon is complementary to and pairs with the corresponding mRNA codon.

Translation The same base pairing rules apply for translation as they did for transcription tRNA anticodon AAA binds with mRNA codon UUU and forms the amino acid phenlythaline

Translation The assembly of a polypeptide begins when a ribosome attaches to the start codon (AUG) on an mRNA transcript. The start codon pairs with the anticodon UAC on a tRNA.

Protein Assembly Amino Acids attach to Translated AA w/ a Covalent bond = Peptide Bond This creates a polypeptide chain = Protein

Overview Transcription Translation mRNA rRNA tRNA Mature mRNA transcripts ribosomal subunits mature tRNA Overview Translation