Download CP Biology Chapter 8 Structure of DNA notes

CP Biology Chapter 8 Structure of DNA, DNA replication and Protein synthesis
DNA is composed of four types of nucleotides
Since the 1920s scientists have known the chemical part of the DNA molecule. DNA is a very long
polymer, or chain of repeating units. The units, or monomers, that make up DNA are called nucleotides.
Each nucleotide has three parts: a phosphate group, a nitrogenous base, and a pentose sugar.
There are four types of DNA nucleotides: cytosine (C), thymine (T), adenine (A), and guanine (G). All of
the nucleotides contain a phosphate group and a deoxyribose sugar. They differ in their nitrogencontaining bases, as shown below.
Notice that thymine and cytosine have nitrogen-containing bases with a single-ring structure. Adenine
and guanine are bases with a double-ring structure. A single molecule of human DNA is made of billions
of nucleotides.
Watson and Crick developed an accurate model of DNA’s three-dimensional structure.
For a long time, scientists hypothesized that DNA in all organisms as made up of equal amounts of the
four nucleotides. Then Erwin Chargaff found that the proportion of the bases differs from organism to
organism. IN the DNA of each organism, the amount of A equals the amount of T, and the amount of G
equals the amount of C.
Then in the early 1950’s, the scientists Rosalind Franklin and Maurice Wilkins used X-rays to make a kind
of photograph of DNA molecules. These photographs did not show what DNA looks like, but they
showed patterns that gave clues about DNA’s structure.
Around the same time, the scientists James Watson and Francis Crick were working together to figure
out DNA structure, too. Based on the work of other scientists, they hypothesized that DNA might have a
spiral, or helix, shape. Watson and Crick saw Franklin’s photos and used the information to complete
their model of DNA structure.
In April 1953, Watson and Crick published their DNA model in a paper in the journal Nature. They found
that nucleotides fit together in a double helix. Two strands of DNA wrap around each other like a
twisted ladder.
Nucleotides always pair in the same way
Each side of the DNA double helix is a long strand of phosphates and sugars, connected by covalent
bonds. The two sides of the double helix are held to each other by hydrogen bonds that form between
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the bases in the middle. Each individual hydrogen bond is weak, but together they are strong enough to
hold the shape of DNA. The bases of the two DNA strands always bond according to the base pairing
rule: T pairs with A, and C pairs with G.
The bases pair in this way because of hydrogen bonds. Notice that A and T form two hydrogen bonds,
whereas C and G form three.
To help remember the rules of base pairing, notice that the letters G and C have a similar shape. Once
you know that G and C pair together, you know that A and T also pair together. If the sequence of bases
on one DNA strand is CTGA, the other DNA strand will be GACT.
Transcription
The central dogma describes how information from DNA gets used to make proteins. The central dogma
involves three processes.
These processes are similar in prokaryotes and eukaryotes, but there are important differences. Here,
we will look at these processes in eukaryotic cells.
RNA stands for ribonucleic acid, RNA, like DNA, is made of nucleotides with three parts: a sugar, a
phosphate group, and a nitrogen-containing base. RNA is different from DNA in three important ways.
1. the sugar in RNA is ribose, not deoxyribose as in DNA. Ribose has one more oxygen atom than
deoxyribose
2. RNA has four bases: A, C, G, and U. RNA has the base uracil (U) instead of thymine (T). Uracil
pairs with adenine.
3. RNA is a single strand of nucleotides, not a double strand like DNA. The single-stranded
structure of RNA allows an RNA molecule to fold up into complex three-dimensional shapes.
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Transcription makes three types of RNA
Transcription is the process of copying a sequence of DNA to produce a complementary strand of RNA.
Only a piece of DNA, or a gene, gets transcribed into RNA, not the whole strand of DNA. Just as DNA
polymerases help with replication, an enzyme called RNA polymerase helps with transcription.
Transcription makes three different types of RNA molecules.
 Messenger RNA (mRNA) carries a message – the instructions that later get turned into a protein
 Ribosomal RNA (rRNA) forms part of ribosomes, the parts of a cell that put amino acids together
in a polypeptide.
 Transfer RNA (tRNA) transfers amino acids in the cytoplasm to the growing polypeptide
The transcription process is similar to replication
Transcription and replication share many similarities. For example, they both involve unwinding the
DNA double helix, and both involve large enzymes called polymerases. But the end results of the two
processes are very different. Replication makes a copy of DNA and transcription makes RNA molecules.
Another difference is that replication happens only once during the cell cycle. Transcription can happen
over and over on the same gene to make many copies of a particular RNA molecule.
Translation
Translation is a process that converts a message from one language into another. For example, a book
may be translated from Spanish into English. Translation happens in cells, too. Cells translate an mRNA
message into amino acids, the building blocks of proteins.
Amino acids are coded by mRNA base sequences
Translation is the process that reads an mRNA message and turns it into a polypeptide. One or more
polypeptides make a protein. An mRNA message can be translated into 20 different amino acids. How
can just four nucleotides – A, U, G, and G – be translated into so many different amino acids? Just as the
26 letters of the alphabet can be used to form many more than 26 words, the four letters of RNA are put
together in different combinations to form many different ‘words”
In the language of the genetic code, a “word” is made of three nucleotides and is called a codon. A
codon codes for an amino acid. Multiple codons can code for the same amino acid. For example, the
codons UCU, UCC, UCA, and UCG all code for serine.
In addition to codons that code for amino acids, there is a start codon that signals the start of
translation. The start codon is AUG, and codes for the amino acid methionine. There are also three stop
codons that signal the end of an amino acid chain.
For words to make sense, they must be read starting at the correct place, or with the correct reading
frame. For example, the sentence “THE CAT ATE THE RAT” makes sense. “T HEC ATA TET HER AT”,
which is made of the same letters, has a different reading frame and does not make sense. The same is
true for codons. There are no spaces between the codons, but he correct start site and the correct
reading frame must be used for the message to make sense.
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Amino acids are linked to become a protein
Remember that transcription makes three kids of RNA – mRNA, rRNA and tRNA. Proteins and rRNA
form ribosomes. Ribosomes and tRNA form the machinery for translating mRNA to make proteins.
A tRNA molecule form an L shape. Different tRNA molecules carry different amino acids. On one end of
a tRNA molecule there is an anticodon that is complementary to a specific mRNA codon. For example, a
tRNA molecule that has the anticodon CCC pairs with the mRNA codon GGG. The same tRNA molecule
also carries the amino acid glycine.
The process of translation happens in the cytoplasm of both prokaryotic and eukaryotic cells. The
illustration above shows the process of translation in just one ribosome. IN a cell, many ribosomes may
translate the same gene at the same time to make many copies of the same protein.
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Mutations
Some mutations affect a single gene, while others affect an entire chromosome
In biology a mutation means a change in an organism’s DNA. A mutation can happen during replication
and affect a single gene. A mutation can also happen during meiosis and affect a whole chromosome.
Genet mutations
There are different types of gene mutations
 A point mutation is when an incorrect nucleotide is put into a DNA molecule during replication.
If the error is not fixed by DNA polymerase, the DNA is permanently changed. For example, the
figure below shows a CTC codon that is changed to a CTA codon. As a result, the wrong amino
acid is added – aspartic acid instead of glutamic acid.

A frameshift mutation is the addition or removal of a nucleotide in the DNA sequence. This
results in a change in the reading frame. Recall that importance of the reading frame from
translation. Think back to the sentence “THE CAT ATE THE RAT”. If the letter E is removed, or
deleted, from the first “THE”, the reading frame is shifted. The result is “THC ATA TET HER AT…”
The reading frame is also shifted if a nucleotide is added, or inserted.
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Chromosomal mutations
Errors in meiosis can result in changes in large parts of a chromosome. Recall that crossing over a
normal process in which chromosomes exchange pieces. Errors in crossing over or in other parts of
meiosis can result in chromosomes with two copies of the same gene. Pieces of non-homologous
chromosomes might even be exchanged.
Mutations may or may not affect phenotype
Whether a mutation affects an organism depends on many different things.
 Type of mutation A point mutation only affects one codon. A frameshift mutation usually has a
bigger effect because it changes the whole reading frame and can affect many codons
 Impact on the amino acid sequence A change in one codon can still have a big effect. For
example, if a codon for an amino acid is changed into a stop codon, transcription would end at
the wrong place. A point mutation may also have no effect. Recall that more than one codon
can code for the same amino acid. For example, CGU, CGC, CGA, and CGG all code for arginine.
A point mutation that changes the last nucleotide of this codon would have no effect on the
resulting amino acid.
 Impact on the resulting protein Some changes might not affect the resulting protein’s shape or
function. Other changes might prevent the protein from functioning. For example, a mutation
could change the active site of an enzyme and prevent the enzyme from binding to its substrate.
 Type of cell Recall that mutations that occur in germ cells can be passed on to offspring.
Mutations in body cells cannot be passed on to offspring.
Mutations can be caused by several factors
Mutations happen. But cells have tools to repair them. For example, DNA polymerase has a
“proofreading” function to fix errors. However, mutations can happen faster than the body’s repair
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system can work. Some mutations are the result of errors that happen normally in the cell. Other
mutations are caused by things in the environment.
 Replication errors DNA polymerase proofreads replication, but a small number of errors are not
fixed. Over time, there are more and more errors. Eventually these mutations affect how the
cell works. There is evidence that a build-up of mutations is a major cause of aging.
 Mutagens Some conditions and substances in the environment can cause DNA mutations –
such as UV light and some chemicals. Things in the environment that can change DNA are called
mutagens
If mutations cause changes that affect the control over cell division, cancer can result.
DNA Replication
Replication copies the genetic information
According to the rules of base pairing, A pairs with T and C pairs with G. If the base sequence of one
strand of DNA is known, the sequence of the other strand is also known. One strand can act as a
template, or pattern, for another strand. During the process of DNA replication, a cell uses both strands
of DNA as templates to make a copy of the DNA.
Recall that your body cells each contain 46 chromosomes made up of DNA. The DNA is copied once
during the cell cycle, in the S phase. After a cell divides, the resulting cells each have a complete set of
DNA.
Proteins carry out the process of replication
DNA does not copy itself. Enzymes and other proteins do the actual work of replication. Here we will
look at the process of replication in eukaryotes. This process is similar in prokaryotes.
First, some enzymes pull apart, or unzip, the double helix to separate the two strands of DNA. Other
proteins keep the strands apart while the strands serve as templates. There are nucleotides floating
around in the nucleus. These nucleotides can pair up, according to the base pairing rules, with the
nucleotides on the open strands. A group of enzymes called DNA polymerases bond the new
nucleotides together. When the process is finished, there are two complete molecules of DNA, each
exactly like the original double strand as shown below.
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Replication is fast and accurate
Your DNA has replicated trillions of times since you grew from a single cell. And DNA replication is
happening in your cells right now. Replication happens very fast. As you can see in the figure below, the
process starts at many different places along a eukaryotic chromosome.
DNA replication is also very accurate. There are very few errors – only about one error per 1 billion
nucleotides. Replication has a built in “proofreading” process. If the wrong nucleotide gets added, DNA
polymerase can find the error, remove the incorrect nucleotide, and replace it with the correct one.
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