DNA Structure & Function - Chapter 10

DNA Structure

• Made from 4 nucleotides (sugar + phosphate group + nitrogenous base)
• Adenine
• Guanine
• Cytosine
• Thymine

KNOW THE STRUCTURE OF A NUCLEOTIDE AND HOW DNA IS READ!!!

• In the cell, DNA is almost always present in a double stranded form.
• Discovered by Franklin, Watson & Crick
• DNA is also wound up in a "Double Helix"
• each side of the DNA ladder is bonded to the other side by ________ _____.
• Complimentary Pairing:
• A bonds to ___
• G bonds to ___
• Chargaff’s rule says that the amount of adenine = _____________ and guanine = _______________

DNA Replication

• Occurs during the S phase of the cell cycle
• One chromatid to two chromatids
• One molecule of DNA to two molecules of DNA
• DNA replication is ______________
• The DNA is unzipped and each strand is used as a template to build a new strand
• DNA replication takes place in only direction (5' -> 3'), meaning that each new piece is made in a different fashion
• leading strand is continuously replicated
• lagging strand is discontinuously replicated in small pieces

• The enzyme responsible for making new DNA is ____  _____________
• This enzyme adds new nucleotides one at a time, using the original strand as a template
• Nucleotides are added in a complementary fashion
• Other enzymes involved include :
• DNA helicase -
• DNA ligase –

DNA Function

• How does DNA direct cellular function?
• How do differences in DNA result in different phenotypes?
• Both questions can be answered by looking at the link between DNA and proteins.

One Gene - One Polypeptide Hypothesis

• Most genes are ___________
• They contain the code for protein production
• Proteins may be either structural or enzymatic
• Different cells produce different proteins
• Individuals with different genotypes also produce slightly different proteins
• Production of different proteins leads to different phenotypes

Protein Synthesis

• DNA acts as a template for manufacturing proteins
• Two step process:
• _____________ = DNA transcribed into RNA
• _____________ = RNA information translated into proteins (polypeptide)

What is a gene?

• Region of DNA containing instructions
• Preceded by a _________
• Within the gene is a code for producing a specific product = polypeptide (protein)
• At the end of the gene is a __________

The Genetic Code

• 4 things you need to know about the genetic code:
• 1 - ______________
• Used by all organisms
• 2 - _____________
• 3 bases read at a time = 64 possible triplets
• Each triplet is called a codon
• 3 - ______________
• Individual bases are only read once
• 4 - ________________
• More than one triplet may code for the same amino acid

Transcription

• Takes information from DNA and copies it to a single-strand of RNA
• RNA is similar to DNA except:
• 1 -
• 2 -
• 3 -

RNA

• Three types of RNA, each made by transcription and then used in translation
• mRNA = __________ RNA
• Contains coding instructions for making proteins
• rRNA = __________ RNA
• Forms the core structure of ribosomes
• tRNA = __________ RNA
• Decodes the message contained in mRNA into amino acids
• Each type of RNA is made by a special enzyme called RNA polymerase

3 steps of transcription

• 1 - Initiation
• RNA polymerase locates the start of a gene (promoter);
• unwinds the DNA;
• and moves downstream to start transcription.
• 2 - Elongation
• RNA polymerase makes a single stranded piece of RNA by adding nucleotides that are complementary to the template DNA.
• 3 - Termination
• At the end of the gene coding region of the DNA is a special combination of bases that signal the end of the gene
• At this point, the RNA polymerase releases the DNA and its single-stranded RNA product

Prokaryotes vs. Eukaryotes

• Prokaryotic organisms (bacteria) proceed directly from transcription to translation
• Eukaryotes separate the processes
• Transcription takes place in the nucleus, translation in the cytoplasm (rough ER)
• The mRNA must be modified before being sent to the cytoplasm
• Addition of a 5’ Cap
• Removal of introns
• Addition of 3’ Poly(A) tail

Translation

• Takes place at the ribosomes
• rRNA already in the cell used to construct ribosomes
• mRNA is read by the ribosomes to create the string of amino acids that make up the proteins
• tRNA carries the individual amino acids and matches them up with their specific codon based on the genetic code
• 1 - Initiation

• 2 - Elongation

• 3 - Termination

DNA Mutations

• Changes in the DNA sequence can affect the final product
• Point mutation = single base changes
• Silent mutation - no change in AA sequence of protein
• Genetic code is redundant, and 3rd base of codon doesn’t usually matter.
• Neutral mutation - change in AA sequence but not change in function of final product
• Missense mutation - change in AA sequence resulting in abnormal protein product
• Nonsense mutation - change from an AA coding codon to a stop codon.
• Stops translation prematurely
• Frameshift mutations
• Insertions of one or two bases
• Alters the reading frame
• Deletions of one or two bases
• Also alters the reading frame

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Terminology:  nucleotide, phosphodiester bond, complimentary bases, Chargaff's rule, adenine, thymine, guanine, cytosine, DNA replication, semiconservative replication, 5' and 3' ends, single stranded DNA, double stranded DNA, antiparallel, DNA polymerase, structural protein, enzymatic protein, transcription, RNA polymerase, mRNA, tRNA, rRNA, initiation, promoter, elongation, coding region, termination, terminator, translation, ribosome, P site, A site, codon, anticodon, amino acid, posttranscriptional modifications, 5' cap, 3' poly-A tail, intron, exon, point mutation, frameshift mutation.