Chapter 8

DNA: The Molecule of Heredity

8-1 DNA

1928 Frederick Griffith: How does bacteria cause pneumonia? Isolated two strains of pneumonia bacteria in

mice; only one of them caused disease Disease strain had smooth colonies; the

harmless strain had rough colonies When injected with the disease strain, mice

got sick and died; w/ the harmless strain, mice stayed healthy

Griffith cont

Next Griffith heat-killed the disease strain and injected it into mice; the mice stayed healthy

Next he mixed the heat-killed strain with the harmless strain and injected mice; mice developed pneumonia and died

Killed bacteria passed on a GENE to the harmless bacteria which then developed the disease causing capability

Griffith’s experiment

DNA

1944 Oswald Avery repeated Griffith’s experiment and determined that DNA stores and transmits genetic information from one generation to the next

Avery repeated the experiment

Avery’s conclusion

DNA

1952 Alfred Hershey and Martha Chase proved the importance of the chemical nature of DNA

Conducted experiments using bacterial viruses

Proved that the viral DNA was the part that caused disease

Hershey and Chase’s experiment

8-2 Chargaff’s Rules

Erwin Chargaff discovered the percentages of nucleotides in DNA

Amounts of guanine = amounts of cytosine Amounts of adenine = amounts of thymine

Rosalind Franklin

1950s used X-ray diffraction to study DNA X-ray created scattered patterns as a

result of the reflection of the X-rays

Francis Crick and James Watson

1953 developed the double helix model of DNA

Used the X-ray diffraction patterns from Franklin’s studies to help determine the structure

Watson and Crick

12-2 Structure and Function of DNA

The components of DNA are deoxyribose , a phosphate group, and a nitrogen base.

Nitrogen base- an organic ring structure that contains one or more atoms of nitrogen.

DNA Molecule

Four Possible Nitrogen Bases

Adenine (A) Guanine (G) Cytosine (C) Thymine (T) This allows for four nucleotides, each

containing one of these four bases These base pairs match as follows: (A) with (T) & (G) with (C)

Chains of Nucleotides

Nucleotides do not exist as individual molecules, they combine to form long chains to produce one molecule

The two chains are held by hydrogen bonds between the bases

This structure resembles a ladder The shape of DNA is known as a double

helix because it looks like a twisted ladder

Importance of Nucleotide Sequence

The genetic material of living things is made of DNA, they are different because of the order of the nucleotides in the DNA strands of the organism

The sequence of nucleotides forms the unique genetic information of an organism

The more closely related the more alike the DNA strands are

8-3 Replication of DNA

DNA replicates because every time a cell divides, it must make a copy of its chromosomes, so each cell can have a complete set

Without replication, species could not survive and individuals could not successfully grow and reproduce

How DNA Replicates

During replication, each strand serves as a pattern to make a new DNA molecule

It begins as an enzyme breaks the hydrogen bond between nitrogen bases that hold the two strands together

The action “unzips” the DNA molecule As the DNA unzips, free nucleotides from

the surroundings in the nucleus, bond to the single strands by base pairing

DNA Replication Cont.

Another enzyme bonds these new nucleotides into a chain

This process continues until the entire molecule has been unzipped and replicated

As a result a new strand is formed that is a complement of one of the original, or parent strand

The result is the formation of two DNA molecules, each identical to the original DNA molecule

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8-4 DNA to Protein

Genetic Code: The sequence of nitrogen base along one of the two strands for the synthesis of proteins

There are 20 different amino acids. But DNA only contains 4 bases.

A single base can’t represent a single amino acid because that system would code for only 4 different amino acids

A sequence of 3 bases provides more than 20 combinations needed to code for all amino acids

Codon: each set of 3 nitrogen bases representing an amino acid

The DNA code is often called a triplet code 64 combinations are possible when a

sequence of three bases is used, 64 different codons are in the genetic code

The order of nitrogen bases in DNA will determine the order of the amino acids in a protein

Codon cont.

For any one codon there can only be one amino acid

The code is said to be universal because the codons represent the same amino acids in all organisms

Transcription-from DNA to RNA RNA structure:

RNA is a nucleic acidDiffers from DNA structure in 3 ways:

1. RNA is usually composed of a single strand rather than a double strand

2. RNA also contains a different type of sugar molecule, ribose instead of deoxyribose

3. RNA also contains four nitrogen bases, but rather than thymine RNA contains Uracil (U)

Making RNA Transcription:

Transcription: the process by which enzymes make an RNA copy of a DNA strand

This process is similar to DNA replication except the main difference is that the process results in the formation of a single-stranded RNA molecule

This RNA copy carries information from the DNA out into the cytoplasm of the cell

RNA by Transcription This is called messenger RNA (mRNA) mRNA carries the information for making a

protein chain Some portions of DNA code for the RNA

that makes up ribosomes, where proteins are synthesized

This type of RNA is called ribosomal RNA (rRNA)

rRNA helps to produce enzymes needed to bond amino acids together during protein synthesis

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8-5 Translation –RNA to Protein Translation: the process of converting

information in a sequence of nitrogen bases in mRNA into a sequence if amino acids that make up protein

This occurs on ribosomes, and involves a third kind of RNA

Transfer RNA (tRNA): brings amino acids to the ribosomes so they can be assembled into proteins

Translating the mRNA code: Correct translation of the code depends

upon the joining of each mRNA codon with the anticodons of the proper tRNA molecules

The end result of translation is the formation of the large variety of proteins that make up the structure of organisms and help them function

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8-7 Genetic Changes

Mutation – A Change in DNA Mutation – any mistake or change in the

DNA sequence Point Mutation – a change in a single base

pair in DNA (EX) The dog chased the car. Changing a single letter in this sentence

changes the entire meaning, a change in a single nitrogen base can change the entire structure of the protein.

The effects of point mutations

Normal

Point mutation

mRNA

ProteinStop

Stop

mRNA

Protein

Replace G with A

Mutations cont.

Frameshift mutation: a mutation when a single base is added or deleted from DNA

EX: a mRNA strand has a new amino acid added to the protein for every codon on the mRNA strand; now a single base is deleted from that strand; this new sequence is now translated into mRNA, but it is out of position by one base; this causes every codon to be out of position by one base; this would cause every amino acid to be changed

Frameshift mutations

mRNA

Protein

Frameshift mutation

Deletion of U

Chromosomal Mutations Mutation affecting gene distribution to

gametes during meiosis, most commonly by deletions, insertions, inversions, or translocations

Some ways they are described as mutating include: parts of the chromosomes are broken off or lost during mitosis or meiosis, chromosomes break and then rejoin incorrectly, or the parts join backwards or even to the wrong chromosome

Effects of chromosomal mutation Occur in living organisms, but they are

especially common in plants Affect the distribution of genes to gametes

during meiosis Gametes that should have a complete set

of genes may end up with extra copies of some genes or a complete lack of certain genes

Few chromosomes mutations are passed on to the next generation because the zygote usually dies

Types of chromosomal mutation Deletions: occur when part of a

chromosome is left out Insertions: occur when a part of a

chromatid breaks off and attaches to its sister chromatid. The result of genes on the same chromosome.

Inversions: occur when part of a chromosome breaks out and is reinserted backwards

Translocation: occur when part of one chromosome breaks off and is added to a different chromosome

Errors in Disjunction

Many chromosomal mutations result from the failure of chromosomes to separate properly during meiosis

Nondisjunction: the failure of homologous chromosomes to separate properly during meiosis

In one form of nondisjunction, 2 kinds of gametes result:

Trisomy: the presence of an extra chromosome

Triploidy: involves a total lack of separation of homologous chromosomes (this condition is rare in animals, but frequent in plants)

Monosomy: absence of a chromosome (these usually do not survive)

Causes of Mutation Spontaneous mutation: mutations that

occur at random Environmental agents also cause

mutations, such as exposure to x-rays, ultraviolet light, radioactive substances, or certain chemicals

Mutations often result in sterility or the lack of normal development in an organism

In human gametes mutations may cause birth defects or in body cells it may cause cancer