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Chapter 12DNA and RNA
What is DNA again?Deoxyribonucleic acidLong double-stranded molecule of
nucleotidesStores genetic code that is transferred
to future generations and cellsCodes for synthesis of proteins
What is a nucleotideA nucleotide has 3 parts:
◦A sugar (deoxyribose for DNA)◦A phosphate group◦A nitrogen base
Nitrogen base pairingThe order of nitrogen bases are what
determines your genetic codeThe order of the bases determines the
proteins that are made4 different bases exist:
◦Purines: Guanine, Adenine Grade A Pure
◦Pyrimidines: Cytosine, ThyminePairing is also called Chargaff’s Rule
Chargaff’s Rule
Result: A=T, G=C
But what is the structure?In the early 1950s Rosalind Franklin
used X-ray diffraction took a “picture” of DNA◦Difficult to tell what the structure is
from the image◦Strands twisted around one another
But what is the structure?James Watson and Francis Crick used
Franklin’s images and their models to determine the correct structure: double helix◦Two strands wound around one
another◦“Twisted ladder”◦Later discovered that hydrogen
bonds hold the two sides of the ladder together
◦Can only from between adenine and thymine, or guanine and cytosine (A=T, G=C)
◦The base pairing explained Chargaff’s Rule
DNA Structure
DNA Structure DNA is very long; E. coli’s contains
4,639,221 base pairs (length =1.6 mm)◦Must fit in an organism 1/1000 its
length◦Must be packed very tightly to fit
Human cell contains almost 100 times the base pairs◦Chromatin consists of DNA packed
around proteins called histones◦These compact together during
mitosis to create our visible chromosomes
DNA Structure
DNA replicationRemember that before a cell divides it
has to copy its DNA◦If we separate each strand, we can
use it to make a copy of the other◦If we have an A, we match it with a
T; if a G, we match it with a C (called complementary)
In prokaryotes, replication begins at one point and proceeds typically in both directions
In contrast, eukaryotes begin replication in hundreds of locations (due to its longer length)
DNA Replication
DNA replicationTwo replication forks formed, each
strand has a new complementary strand added◦TACGTT ATGCAA◦Results in two identical DNA
molecules◦One strand is new, the other old
DNA was unzipped (breaking of hydrogen bonds)◦DNA polymerase (enzyme) joins
individual nucleotides to produce a DNA molecule
What is RNA?Ribonucleic acidIntermediate molecule between DNA
and proteinsDiffers from DNA in 3 ways:
◦Single stranded (vs. double)◦Ribose instead of deoxyribose◦Utilizes uracil instead of thymine (A-
U, G-C)
Type of RNAThere are 3 types of RNA:
◦messenger RNA (mRNA)- carries copies of the protein instructions
◦ribosomal RNA (rRNA)- makes up the ribosome, along with several proteins
◦transfer RNA (tRNA)- take the appropriate (base pairing anti-codon to codon) amino acid to the ribosome
Types of RNA
Protein synthesis: how we make proteins?Two processes or steps:
◦Transcription- mRNA is made from the DNA and travels to the cytoplasm to find a ribosome
◦Translation- tRNA brings to the ribosome the amino acids to build the primary structure of a protein and the result is a free polypeptide that will then fold up into the shape of the protein
Remember: amino acids are held together by peptide bonds
Transcription
TranscriptionRNA polymerase binds to DNA and
separates the strandsRNA polymerase uses one strand to
serve as a template to create a strand of RNA
RNA polymerase only binds to regions of DNA called promoters (landing pad)
The RNA is next edited, removing the introns, while leaving the exons (exons=expressed)
RNA Splicing
TranslationProteins are made by joining amino acids
(1 of 20) into long chains (polypeptides)◦How do 4 letters lead to so many
different amino acids?◦Read 3 letters at a time, called a codon◦UCGCACGGU◦UCG-CAC-GCU◦Serine-Histidine-Glycine◦Some codons code for the same amino
acid◦AUG is the start codon
TranslationCodon – 3 nucleotides that code for an
amino acid. Found on the mRNA.Anticodon - Fit the codon. Found on
the tRNA
DNA: master plan; never brought to work site
mRNA: blue prints made from master plan
Ribosomes: building sitetRNA: trucks unloading materials
Codons
Codons
Translation
Transcription and Translation
MutationsMutations – changes made in the genetic
material; can occur during copyingPoint mutation – effect only one
nucleotide. Frameshift mutation – an insertion or
deletion that effects the remaining string of nucleotides.
Chromosomal mutations – changes in the number or structure of chromosomes.
Polyploidy – when an organism has extra sets of chromosomes.
MutationsTHE FAT CAT ATE THE RAT
◦Delete H (Frameshift / deletion)TEF ATC ATA TET HER AT
◦Change H to L (Point mutation)TLE FAT CAT ATE THE RAT
MutationsDNA: TAC GCA TGG AATRNA: AUG CGU ACC UUAAA: Met Arg Thr Leu
◦SubstitutionDNA: TAC GTA TGG AATRNA: AUG CAU ACC UUAAA: Met His Thr Leu
◦ InsertionDNA: TAT CGC ATG GAA TRNA: AUA GCG UAC CUU AAA: Ile Ala Tyr Leu
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