Download 8.2 Structure of DNA - Perry Local Schools

8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
8.2 Structure of DNA
• We love D N A
• Made of nucleotides
• Sugar, Phosphate and a
Base
• Bonded down one Side
• Adenine and Thymine
• Make a Lovely Pair
• Cytosine without
Guanine
• Would feel very bare
•O-O-O deoxy-ribo-nucleic acid
•R-N-A is ribo-nucleic acid
8.2 Structure of DNA
• DNA is made up of a long chain of nucleotides.
• Each nucleotide has three parts.
– a sugar – deoxyribose
– phosphate group
– a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
8.2 Structure of DNA
• The nitrogen containing bases
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G))
* Purines – have 2 rings = Adenine & Guanine
* Pyrimidines – have 1 ring = Cytosine & Thymine
8.2 Structure of DNA
Watson and Crick determined the three-dimensional
structure of DNA
• 1953
• Two nucleotide chains that wrap
around each other to form a
double spiral (double helix)
- Temperature liable – a change in
T can break apart the DNA strand
8.2 Structure of DNA
• Rosalind Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA was a
double helix of even width.
– Chargaff’s - Complementary Base Pairing - A=T
and C=G.
8.2 Structure of DNA
Nucleotides always pair in the same way.
• Because a pyrimidine (single ring)
pairs with a purine (double ring), the
helix has a uniform width.
• A-T
• C-G
G
C
A T
8.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
8.3
DNA Replication
8.2 Structure
of DNA
KEY CONCEPT
DNA replication copies the genetic information of a
cell.
8.3
DNA Replication
8.2 Structure
of DNA
Replication copies the genetic information.
• The rules of base pairing direct
replication.
• A-T
• G-C
• DNA is replicated during the
S stage of interphase.
8.3
DNA Replication
8.2 Structure
of DNA
Proteins (Enzymes) carry out the process of replication.
• DNA serves as a template.
1. Helicase – enzyme that breaks the Hydrogen bonds
between the bases
• Replication Fork – point at which the two chains
separate (last bond broken)
nucleotide
The DNA molecule unzips
in both directions.
8.3
DNA Replication
8.2 Structure
of DNA
2. New complimentary nucleotide bases pair up on both
sides of old DNA template
3. DNA polymerase (enzyme) forms new Hydrogen bonds
between the nucleotides
new strand
nucleotide
DNA polymerase
8.3
DNA Replication
8.2 Structure
of DNA
• Two new exact copies of DNA are formed, each with an
original strand and a newly formed strand.
original strand
Two molecules of DNA
new strand
8.3
DNA Replication
8.2 Structure
of DNA
Replication is fast and accurate.
• DNA replication starts at many points in eukaryotic
chromosomes.
There are many origins of replication in eukaryotic chromosomes.
• Mutation – change in the nucleotide sequence
• DNA polymerases can find and correct errors.
• Error Rate - one error per 1 billion nucleotides
8.2
of DNA
8.4 Structure
Transcription
• RNA differs from DNA in three major ways.
1. RNA has a ribose sugar.
2. RNA has uracil instead of thymine.
U -A
3. RNA is a single-stranded structure.
8.2
of DNA
8.4 Structure
Transcription
Three types of RNA.
Mesenger RNA (mRNA) – single uncoiled chain–
• carries genetic information from the DNA in the nucleus to
the cytoplasm
Transfer RNA (tRNA) – single chain of about 80
• RNA nucleotides folded into a hairpin shape –
• binds to specific amino acids
Ribosomal RNA (rRNA) – makes up the ribosomes where
proteins are made
8.2
of DNA
8.4 Structure
Transcription
• Process of copying DNA into mRNA
• RNA polymerase – starts RNA transcription by binding to
specific regions of DNA
– Promoters
• RNA polymerase breaks H-bonds and makes H-bonds
between the DNA bases
• One chain is used as a template to build RNA
(mRNA=transcript)
• Transcription continues one nucleotide at a time until the
RNA polymerase reaches a DNA region
– termination signal
start site
transcription complex
nucleotides
8.2
of DNA
8.4 Structure
Transcription
The transcription process is similar to replication.
• The two processes have different end results.
– Replication copies
all the DNA;
transcription copies
a gene.
one
gene
growing RNA strands
DNA
8.2
of DNA
8.5Structure
Translation
KEY CONCEPT
Translation converts an mRNA message into a
protein.
8.2
of DNA
8.5Structure
Translation
Amino acids are coded by mRNA base sequences.
• Codon – 3 nucleotides of mRNA
• AUG = start
• UAA, UAG, UGA = stop
codon for
methionine (Met)
codon for
leucine (Leu)
8.2
of DNA
8.5Structure
Translation
• The genetic code matches each codon to its amino acid
8.2
of DNA
8.5Structure
Translation
• tRNA – transports amino acids to the ribosomes
• Anticodon – tRNA sequence of 3 nucleotides
– complementary to an mRNA codon.
8.2
of DNA
8.5Structure
Translation
Ribosomes that are attached to the endoplasmic reticulum
build proteins for use outside cell
Ribosomes that are free floating make proteins for use
inside cell
8.2
of DNA
8.5Structure
Translation
• For translation to begin - Ribosomes attaches to a start codon
on mRNA (AUG)
• Start codon pairs with the anticodon on tRNA (UAC)
• codes for the first amino acid – methionine – may be
removed later if not needed
8.2
of DNA
8.5Structure
Translation
• Amino acids are bonded together with peptide bonds
8.2
of DNA
8.5Structure
Translation
– Once the stop codon is reached, the ribosome
releases the protein
8.2
of DNAand Regulation
8.6 Structure
Gene Expression
KEY CONCEPT
Gene expression is carefully regulated in both
prokaryotic and eukaryotic cells.
8.2
of DNAand Regulation
8.6 Structure
Gene Expression
• A promotor is a DNA segment that allows a gene to be
transcribed.
• An operator is a part of DNA that turns a gene “on” or ”off.”
– The lac operon was one of the first examples of gene
regulation to be discovered.
– The lac operon has three genes that code for enzymes
that break down lactose.
8.2
of DNAand Regulation
8.6 Structure
Gene Expression
• RNA processing is also an important part of gene regulation
in eukaryotes.
– Introns are nucleotides that are removed and exons
nucleotides that are spliced together.
8.2
of DNAand Regulation
8.6 Structure
Gene Expression
Coding DNA (genes)  make proteins
• Humans = 20,000
Non-coding DNA (genes)  make RNA (transcribed but never
translated)
• Human = 500
Human Total = ~ 20,500 genes