DNA

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DNA

• Chapter 11

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The main nucleic acids

• There are 2 main nucleic acids
• 1. DNA Deoxyribonucleic Acid
• 2. RNA Ribonucleic Acid

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What is DNA?

• Deoxyribonucleic Acid (DNA) is the genetic
  enzymes that is critical for an organisms
  function because they control the chemical
  reactions needed for life

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The structure of DNA

• DNA is a polymer made of repeating subunits
  called nucleotides
• Nucleotides have three parts 1. a simple sugar
  (deoxyribose), 2. a phosphate group, and 3. a
  nitrogenous base

Nitrogen base
Phosphate group
Sugar (deoxyribose)
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Parts of the Nucleotide

• The simple sugar in DNA, called deoxyribose (dee
  ahk sih RI bos), gives DNA its namedeoxyribonucle
  ic acid.
• The phosphate group is composed of one atom of
  phosphorus surrounded by four oxygen atoms

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Parts of the Nucleotide

• A nitrogenous base is a carbon ring structure
  that contains one or more atoms of nitrogen.
• In DNA, there are four possible nitrogenous
  bases adenine (A), guanine (G), cytosine (C),
  and thymine (T).

Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
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Parts of the Nucleotide

• Nucleotides join together to form long chains,
  with the phosphate group of one nucleotide
  bonding to the deoxyribose sugar of an adjacent
  nucleotide
• The phosphate groups and deoxyribose molecules
  form the backbone of the chain, and the
  nitrogenous bases stick out like the teeth of a
  zipper
• Each nitrogen base is held together by a hydrogen
  bond

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The structure of DNA
Because DNA is composed of two strands twisted
together, its shape is called double helix
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Watson Crick

• In 1953, Watson and Crick proposed that DNA is
  made of two chains of nucleotides held together
  by nitrogenous bases
• Watson and Crick also proposed that DNA is shaped
  like a long zipper that is twisted into a coil
  like a spring

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The importance of the nucleotide sequence

• The sequence of nucleotides forms the unique
  genetic information of an organism. The closer
  the relationship is between two organisms, the
  more similar their DNA nucleotide sequences will
  be.

Chromosome
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DNA Replication

• Before a cell can divide by mitosis or meiosis,
  it must first make a copy of its chromosomes
• The DNA in the chromosomes is copied in a process
  called DNA replication
• Without DNA replication, new cells would have
  only half the DNA of their parents

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Replication of DNA
DNA
Replication
Replication
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Copying DNA

• DNA is copied during interphase prior to mitosis
  and meiosis.
• It is important that the new copies are exactly
  like the original molecules

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Copying DNA
New DNA molecule
Original DNA Strand
New DNA molecule
Free Nucleotides
New DNA Strand
Original DNA Strand
Original DNA
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The importance of DNA

• Scientists use nucleotide sequences to determine
• 1. evolutionary relationships among organisms
• 2. to determine whether two people are related
• 3. to identify bodies of crime victims.

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RNA

• Section 2

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3 Parts of the RNA nucleotide

• Ribose (sugar)
• Phosphate Acid
• 4 Nitrogen Bases
• 1. A- Adenine
• 2. U- Uracil (replaces Thymine)
• 3. C- Cytosine
• 4. G- Guanine

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RNA vs DNA

• RNA like DNA, is a nucleic acid.
• RNA structure differs from DNA structure in three
  ways.
• RNA is single strandedDNA is double stranded

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RNA vs DNA

• 2. The sugar in RNA is ribose - DNAs sugar is
  deoxyribose

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RNA vs DNA

• Both DNA and RNA contain four nitrogenous bases,
  but rather than thymine, RNA contains a similar
  base called uracil (U).

Uracil forms a base pair with adenine in RNA,
just as thymine does in DNA.
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RNA provides instruction for making protein

• RNA takes instructions from DNA on the assembly
  of proteins, thenamino acid by amino acidthey
  assemble the protein.
• There are three types of RNA that help build
  proteins.
• 1. mRNA messenger RNA
• 2. tRNA transfer RNA
• 3. rRNA ribosomal RNA

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How it all works

• Messenger RNA (mRNA), brings instructions from
  DNA in the nucleus to the cells factory floor,
  the cytoplasm
• On the factory floor, mRNA moves to the assembly
  line, a ribosome.
• The ribosome, made of ribosomal RNA (rRNA), binds
  to the mRNA and uses the instructions to assemble
  the amino acids in the correct order

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THEN

• Transfer RNA (tRNA) is the supplier. Transfer
  RNA delivers amino acids to the ribosome to be
  assembled into a protein
• In the nucleus, enzymes make an RNA copy of a
  portion of a DNA strand in a process called
  transcription.

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Transcription vs Replication

• The main difference between transcription and DNA
  replication is that transcription results in the
  formation of one single-stranded RNA molecule
  rather than a double-stranded DNA molecule

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The Genetic Code

• The nucleotide sequence transcribed from DNA to a
  strand of messenger RNA acts as a genetic
  message, the complete information for the
  building of a protein
• As you know, proteins contain chains of amino
  acids. You could say that the language of
  proteins uses an alphabet of amino acids
• A code is needed to convert the language of mRNA
  into the language of proteins

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Codon

• Biochemists began to crack the genetic code when
  they discovered that a group of three nitrogenous
  bases in mRNA code for one amino acid. Each group
  is known as a codon
• Sixty-four combinations are possible when a
  sequence of three bases is used thus, 64
  different mRNA codons are in the genetic code

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Facts about Codons

• Some codons do not code for amino acids they
  provide instructions for making the protein
• More than one codon can code for the same amino
  acid.
• For any one codon, there can be only one amino
  acid

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The Genetic Code

• All organisms use the same genetic code
• This provides evidence that all life on Earth
  evolved from a common origin

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Translation from mRNA to Protein

• The process of converting the information in a
  sequence of nitrogenous bases in mRNA into a
  sequence of amino acids in protein is known as
  translation
• Translation takes place at the ribosomes in the
  cytoplasm
• In prokaryotic cells, which have no nucleus, the
  mRNA is made in the cytoplasm
• In eukaryotic cells, mRNA is made in the nucleus
  and travels to the cytoplasm
• In cytoplasm, a ribosome attaches to the strand
  of mRNA

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Role of Transfer RNA

• For proteins to be built, the 20 different amino
  acids dissolved in the cytoplasm must be brought
  to the ribosomes
• This is the role of transfer RNA

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Amino acid
Each tRNA molecule attaches to only one type of
amino acid.
Chain of RNA nucleotides
Transfer RNA molecule
Anticondon
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tRNA
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The role of transfer RNA
Ribosome
mRNA codon
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The role of tRNA

• Usually, the first codon on mRNA is AUG, which
  codes for the amino acid methionine
• AUG signals the start of protein synthesis
• When this signal is given, the ribosome slides
  along the mRNA to the next codon

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The role of transfer RNA
Methionine
tRNA anticodon
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The role of transfer RNA
A new tRNA molecule carrying an amino acid pairs
with the second mRNA codon.
Alanine
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The role of transfer RNA
The amino acids are joined when a peptide bond is
formed between them.
Methionine
Alanine
Peptide bond
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The role of transfer RNA
A chain of amino acids is formed until the stop
codon is reached on the mRNA strand.
Stop codon
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Mutations

• Section 3

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Mutations in reproductive cells

• By changing the sequence of nucleotides within a
  gene in a sperm or an egg cell
• The altered gene would become part of the genetic
  makeup of the offspring
• May produce a new trait or a protein that does
  not work correctly or a protein that is
  nonfunctional
• The embryo may not survive
• In some rare cases a gene mutation may have
  positive effects

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Mutations in Body Cells

• If the cells DNA is changed, (by a gamma ray)
  this mutation would not be passed on to offspring
  but could cause problems for the individual
• Damage to a gene may impair the function of the
  cell
• When that cell divides, the new cells also will
  have the same mutation
• Some mutations of DNA in body cells affect genes
  that control cell division resulting in cancer

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Point Mutation

• A point mutation is a change in a single base
  pair in DNA
• A change in a single nitrogenous base can change
  the entire structure of a protein because a
  change in a single amino acid can affect the
  shape of the protein.

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example

• Normal
• THE DOG BIT THE CAT
• Mutation
• THE DOG BIT THE CAR

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The effects of point mutations
mRNA
Normal
Protein
Stop
Replace G with A
Point mutation
mRNA
Protein
Stop
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Frameshift Mutation

• What would happen if a single base were lost from
  a DNA strand?
• Frameshift mutation is an addition or deletion of
  a base in a DNA strand

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Frameshift Mutation
Deletion of U
mRNA
Protein
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Causes of Mutations

• Could be spontaneous, or caused from
  environmental factors
• Any agent that can cause a change in DNA is
  called a mutagen
• Radiations
• Chemicals
• High Temperatures