Download deoxyribonucleic acid Deoxyribose – simple sugar in DNA DNA is

The Components and Structure of DNA
•DNA – deoxyribonucleic acid
•Deoxyribose – simple sugar in DNA
•DNA is made up of nucleotides
•Nucleotide – made of simple sugar, phosphate and a
nitrogen base
•4 Nitrogen Bases in DNA
1. Adenine
purines – double ring
2. Guanine
3. Cytosine
4. Thymine
pyrimidines – single ring
•
Nucleotides join together to form long chains
•
Phosphate and deoxyribose form the
backbone of the chain (sides of the ladder)
•
Nitrogen bases are the steps of the ladder
•
The amount of adenine equals thymine
•
The amount of guanine equals cytosine
Fig. 13.2, p. 215
Slide 2
phosphate
deoxyribose
nitrogen base
hydrogen
bond
nucleotide
History of DNA
•Rosalind Franklin and Maurice Wilkins worked on the
structure of DNA based on x-ray crystallography
(pictures of DNA by x-rays).
•James Watson and Francis Crick were also working on
the structure of DNA
•These scientists were trying to be the first to discover
and prove the structure of DNA because they knew it
would be one of the most important discoveries in the
20th century.
•
1953 Watson and Crick proposed that DNA is made of
two chains of nucleotides joined together by nitrogen
bases
•
Two bases are held together by hydrogen bond
•
Two strands are complementary
•
Complementary base pairing
•
•
A and T
•
C and G
DNA is shaped in a double helix
A-A-G-C-T-T-G-C-C-A-T-C-A-G-T-G-G-A-C
T-T-C-G-A-A-C-G-G-T-A-G-T-C-A-C-C-T-G
The Importance of Nucleotide Sequences
•All living organisms are composed of A, T, C, G
•How can organisms be so different from each other if their
genetic material is made of the same four nucleotides?
•Differences in organisms are from the sequence of the four
different nucleotides and how many nucleotides
•The closer the relationship between two organisms the
greater the similarity in their order of DNA nucleotides
•Scientists use nucleotide sequences to
•
Determine evolutionary relationships among organisms
•
Determine whether two people are related
•
Determine if DNA from a crime scene matches the DNA
of a suspected criminal
Replication of DNA
•Before a cell can divide by mitosis or meiosis it must
first make a copy of its chromosomes
•DNA Replication – DNA is copied
•All organisms undergo replication
How DNA Replicates
•
During replication each strand serves as a pattern to
make a new DNA molecule
•
The end result is the formation of two DNA molecules
that are identical (duplicated chromosome)
•
Steps of Replication
1. Enzyme, DNA helicase, breaks the hydrogen
bonds between nucleotides, this “unzips” the DNA
molecule
2. Free nucleotides in the nucleus bond to the single
strands. Enzyme, DNA polymerase glues the new
strands together
3. This continues until the entire molecule has been
unzipped and replicated
4. Each new strand formed is a complement of one
of the originals or parent strand.
5. Two DNA molecules that are identical
6. Proofreading enzymes check the strand for
mistakes and repair enzymes fix any mistakes that
occur
From DNA to Proteins
•
Occurs in two steps – Transcription and Translation
Genes and Proteins
•
Proteins are key cell structures and regulators of cell
functions
•
The sequence of nucleotides makes amino acids
•
Sequence of amino acids make proteins
RNA
•RNA – ribonucleic acid
•Differences Between DNA and RNA
DNA
RNA
Double Stranded
Single Stranded
Sugar is Deoxyribose
Sugar is Ribose
Adenine, Guanine, Cytosine,
Thymine
Adenine, Guanine, Cytosine,
Uracil
DNA cannot leave the nucleus
RNA can leave the nucleus
DNA – A-C-G-T-G-A-A-G-C-T-G-T-A-C-A-G-T-C
RNA – U-G-C-A-C-U-U-C-G-A-C-A-U-G-U-C-A-G
•
3 Types of RNA that help to build proteins
1. Messenger RNA (mRNA) – takes information from
the DNA in the nucleus to the ribosome
2. Ribosomal RNA (rRNA) – what ribosomes are made
of
3. Transfer RNA (tRNA) – transports amino acids to
the ribosome
Transcription
•
Transcription – make an RNA copy of a portion of a
DNA strand
•
The process of transcription is similar to replication
except
•
Transcription makes a single strand of RNA
•
Does not transcribe the entire strand of DNA
•
Steps of Transcription From DNA to RNA
1. An enzyme, RNA polymerase, unzips the
molecule of DNA
2. As the DNA molecule unzips, RNA polymerase
adds on RNA nucleotides to one strand of DNA.
3. Transcription continues until RNA polymerase
reaches a special stop sequence - AAAAAAAA
4. mRNA molecule breaks away and leaves the
nucleus
The Genetic Code
•
Proteins are built from amino acids
•
20 different amino acids
•
Codon – each set of 3 nitrogen bases represents an
amino acid
•
The order of nitrogen bases in DNA can determine the
type and order of amino acids in a protein
•
64 different combinations
•
61 code for amino acids
•
3 signal to stop protein synthesis
•
More than 1 codon can code for the same
amino acid
•
Start codon is AUG (methionine)
•
Stop codons are UAA, UAG, UGA
Translation
From mRNA to Protein
•Translation – the process of converting mRNA into a
sequence of amino acids
•Takes place at the ribosome
•
•
tRNA
•
Transfers amino acids to ribosome
•
One end of molecule carries amino acid
•
Other end carries anti-codon which complements
the codon
Ex: mRNA – A-C-A
tRNA – U-G-U
Translating the mRNA Code
MOVIE
Steps of Translation
1. The first codon of the mRNA strand attaches to a
ribosome
2. tRNA molecules carrying a specific amino acid
approach the ribosome
3. tRNA anticodon pairs with mRNA codon
4. The first codon on mRNA is AUG which codes for
amino acid methionine. AUG is the start codon for protein
synthesis
5. A new tRNA molecule carrying an amino acid will pair
with the next mRNA codon
6. As the process continues a chain of amino acids is
made until it reaches a stop codon on the mRNA; UAA,
UAG, UGA
DNA
T–A–C–A–G–G–T–C–G–T–T–A–C–G–G–A–C–T
mRNA
A–U–G–U–C–C–A–G–C–A–A–U–G–C–C–U–G–A
tRNA
U–A–C–A–G–G–U–C–G–U–U–A–C–G–G–A–C–U
Amino
Meth.
Acids
Serine
Serine Aspar.
Alanine Stop
Mutations
Mutation: A Change in DNA
•
Mutation – any change in the DNA sequence
•
Mutations in Reproductive Cells
•
Mutation in egg or sperm can be passed on to
offspring
•
Sometimes the mutation is so severe that the
embryo does not survive
•
In rare cases a gene mutation may have positive
effects
•
Mutations in Body Cells
•
This mutation would not be passed on to
offspring
•
But the mutations can cause harm to the
individual
DNA Mutations
1. Point Mutation
•
A change in a single base pair in DNA
•
A change in a single letter can change the
amino acid, thus changing the protein made
2. Frameshift Mutation
•
A single base is added or deleted from DNA
•
Can cause nearly every amino acid in the
protein to be changed.
Chromosomal Mutations
•
Chromosomal Mutations – changes in the structure
of chromosomes
•
They occur in all living organisms, but they are
especially common in plants
•
Chromosomal mutations are rarely passed on to the
next generation because:
•
The zygote usually dies
•
The mature organism is usually sterile
•
4 Types of Chromosomal Mutations
1. Deletion – a fragment of a chromosome breaks
off, it can be lost
2. Duplication – achromosome fragment attaches to
its homologous chromosome, which will then
carry two copies of a gene
3. Inversion – fragment reattaches to the original
chromosome in the reverse orientation
4. Translocation – a fragment may join a
nonhomologous chromosome
Causes of Mutations
•
Spontaneous Mutations – a mistake in base pairing
during DNA replication. It occurs at random.
•
Mutagen – any agent that can cause a change in DNA
•
Ex. Chemicals, radiation, high temperatures
Repairing DNA
•
When mistakes do occur repair mechanisms fix
mutations
•
Proofreading Enzymes – reads the DNA strand and
checks it for mistakes
•
Repair Enzymes – fixes any mistakes in the DNA
strand
Mistakes in Meiosis
•
Sometimes accidents occur during meiosis and
chromosomes fail to separate correctly
•
Nondisjunction – failure of homologous
chromosomes to separate
•
During meiosis I one chromosome from each pair is
supposed to move to opposite poles but
occasionally both chromosomes of a pair move to
the same pole
•
Trisomy – 1 extra chromosome (47)
•
•
Ex: extra chromosome on pair number 21 – Down
syndrome
Monosomy – missing 1 chromosome (45)
•
Ex: missing chromosome on pair number 23 –
Turner syndrome
•
Tetraploid – 2 extra chromosomes (48)
•
Polyploids – organisms with more than the usual
number of chromosome sets
•
Is rare in animals and almost always results in
death.