Chapter 11

Quiz #8: February 13th

You will distinguish between the famous

scientists and their contributions towards DNA

You will demonstrate replication, transcription,

and translation from a sample strand of DNA

You will identify mutations by type and result

When trying to figure out what our genes were

made of, most scientists always assumed they

were proteins.

Proteins were discovered in 1838; Nucleic acids

were discovered in 1871.

The function of proteins was discovered in 1926.

Nucleic acids not until 1952.

There are 20 different amino acids that make

proteins. There are only 4 different nucleotides

that make up nucleic acids.

There’s more of a variety of proteins and

we’ve known about them longer.

Alfred Hershey and Martha Chase devised an

experiment to answer this question in 1952.

Proteins contain sulfur, but nucleic acids don’t.

Nucleic acids contain phosphorus, but proteins

don’t.

First the two scientists put radioactive isotopes

of both phosphorus and sulfur into a virus.

The radiation would show up under a special light,

and will shine a different color for phosphorus and

sulfur.

After this, they let the virus infect an E. coli

cell, which reproduced over and over again.

After the E. coli reproduced and grew, Hershey and Chase shined radioactive detectors over the cells.

They found that the color for phosphorus appeared, but not for sulfur.

This proved that when the virus reproduced, they passed on phosphorus, not sulfur.

Since only nucleic acids have phosphorus, nucleic acids must have been what was passed on.

Conclusion: Genes were made of nucleic acids, not proteins.

All of a sudden, people were desperate to learn more about our genetic material: nucleic acids.

There are only 5 different types of nucleic

acids.

Adenine, Thymine, Guanine, Cytosine, and Uracil

Each nucleotide contains three parts

A 5-carbon ribose sugar, which is the structural

backbone of nucleic acids

A phosphate molecule, which links nucleotides

together

A nitrogenous base, which is the genetic code.

The different nitrogenous bases give each

nucleotide it’s specific name.

Around the same time as Hershey and Chase,

Erwin Chargoff noticed an interesting trend

with nucleotides.

Chargoff separated all the DNA in a

chromosome into individual nucleotides.

He then weighed each nucleotide to see how

much of each was in a chromosome.

Though the numbers were always different,

the amount of adenine was always similar to

the amount of thymine, and the amount of

guanine was similar to the amount of

cytosine

Species A T G C

Bacillus Subtillus (Bacillus bacteria) 28.4 29.0 21.0 21.6

Escherichia coli (E. coli) 24.6 24.3 25.5 25.6

Neurospora crassa (Bread mold) 23.0 23.3 27.1 26.6

Zea mays (Corn) 25.6 25.3 24.5 24.6

Drosophila melanogaster (Fruit fly) 27.3 27.6 22.5 22.5

Homo Sapiens (Human) 31.0 31.5 19.1 18.4

One year after Hershey and Chase, James

Watson and Francis Crick made what is

considered one of the greatest discoveries in

science: they decoded the structure of DNA.

Using photographs from their colleague,

Rosalind Franklin, the amount of each

nucleotide from Chargoff, and tinker toys,

they built large models showing how to build

a DNA strand.

Watson and Crick proved that even though

there are only four nucleotides, they could

be rearranged and linked billions of times to

create long sequences.

Thus, genes could be created by linking

hundreds, thousands, or tens of thousands of

nucleotides together.

There are three processes we will discuss in

this chapter. The first is DNA replication.

DNA replication is how cells copy their DNA

These copies will be used by each cell during

mitosis and meiosis.

To learn replication, you must first learn the

structure of DNA chains

DNA is described as a double helix, because

it is two sequences of DNA that wrap around

each other.

These sequences attach to each other at the

nucleotides using hydrogen bonds.

Adenines always attach to Thymines

Guanines always attach to Cytosines.

(This explains Chargoff’s rule)

Because these pairs always attach to each

other, each strand of DNA can be used as a

template, or guide, for building a new strand

The first step in DNA replication is that an

enzyme has to break the hydrogen bonds

between each strand.

After breaking the first hydrogen bond, the

enzyme continues down the strand like a

zipper.

Meanwhile, the endoplasmic reticulum has

been building new nucleotides.

These nucleotides are carried by enzymes to

the nucleus to be attached to the unzipped

strand.

While the strands unzip, enzymes attach new

nucleotides to each strand.

The enzymes know which nucleotides to use

because each nucleotide can only pair with

it’s partner—no one else.

The result: each new DNA strand has one old

copy of DNA and one new copy of DNA.

This process of using an old strand to build a

new is called semi-conservative replication.

Structurally, not much is

different between DNA and

RNA.

DNA stands for

DeoxyriboNucleic Acid

RNA stands for RiboNucleic

Acid

The only difference in

structure is that DNA is

missing an oxygen atom

There are other differences with how DNA

and RNA is used

DNA is double stranded, RNA is single stranded

(the nucleotides do not have to pair up)

RNA does not contain Thymine. Instead, it

contains a unique nucleotide called Uracil.

DNA’s only form is double helix. RNA comes in

many forms. You will learn about two of these

forms: messenger RNA, and transfer RNA

The DNA is the main blueprint. RNA not only

are copies of the blueprint, but do most of

the constructing as well.

Last chapter we talked about how genes are

passed on. This chapter, we will talk about

how our body reads genes and turns them

into our traits.

To do this, we will introduce the second

process of this chapter: DNA Transcription.

Transcription is the process of creating an

RNA sequence using a DNA template.

This RNA sequence will then be carried out of

the nucleus and to a ribosome to build a

protein.

Transcription begins similar to Replication. An

enzyme breaks the hydrogen bonds between

the DNA strands and begins to unzip a section

of DNA

This section contains only the necessary genetic

information for making a specific protein.

As it unzips, enzymes attach new nucleotides

to the DNA segment

Guanine pairs with Cytosine (same as replication)

Adenine pairs with URACIL (not Thymine this time)

When the section of nucleotides is completed,

the RNA strand breaks off of the DNA strand.

This section of RNA is called Messenger RNA (or

mRNA) because it will be the messenger carrying

the genetic information to the ribosome.

Another enzyme reattaches the DNA strand, and

the process of transcription is complete.

Before moving on, the mRNA strand first gets

modified by the cell

Sections of the mRNA are removed by an enzyme

and returned to the nucleus.

The sections that are removed are called introns.

The remaining sections, called exons, are what

get expressed (read) by ribosomes

Why introns and exons?

The short answer is…who knows?

Introns create redundancy, which helps reduce the

likelihood that a mutation will cause a problem

If 100% of the nucleotides are turned into a protein, then a

mutation will cause a problem 100% of the time.

If only 100 nucleotides out of 10,000 code for a gene, the

likelihood of a mutation hitting those specific 100

nucleotides is very small.

Introns allow for more variety of gene sequences

Take the word “hearth”.

From this word, you get the words “he,” “ear,” “art,”

“heart,” and “earth,” depending on which letters you cut

out.

The final process to learn this chapter is

translation.

Translation is the process of converting a

strand of mRNA into an amino acid sequence

for a protein

Translation occurs at ribosomes

Remember chapter 7? What is the role of

ribosomes?

The first step involves the second form of

RNA: Transfer RNA (tRNA)

The tRNA have two parts.

On one end, they have what is called an

anticodon.

A codon is a sequence of three nucleotides in

mRNA

An anticodon is a sequence that pairs with a

codon

On the other end, they are holding on to an

amino acid.

The beginning of the sequence of mRNA is

read by multiple tRNA’s until one is found

whose anticodon matches the mRNA’s codon

Once the right tRNA has latched onto the

mRNA, another tRNA will latch onto the next

3-nucleotide codon sequence.

This process will continue until the entire

strand has been covered

While tRNA’s match their anticodon’s to the

mRNA’s codons, the amino acids on the other

ends of the tRNA’s are lining up too.

Another enzyme will remove the amino acids

from the tRNA and attach them to each

other.

The type of bond that holds the amino acids

together is called a peptide bond

Once the amino acid sequence is completed,

the amino acids will fold together and form a

protein

How does the mRNA know that the amino acid sequence it is coding for is the right one?

Each tRNA has a specific anticodon. That anticodon will ALWAYS go with a specific amino acid

Each three-nucleotide sequence on the mRNA can therefore only code for a specific amino acid. One sequence always indicates the *Start*

position, and three sequences indicate the *Stop* positions

The four possible nucleotides for RNA are

Uracil (U)

Cytosine (C)

Adenine (A)

Guanine (G)

Name some possible 3-nucleotide codons that

we will test to see what amino acid they

code for.

The human genome contains between

20,000-40,000 genes. The sequence of DNA to

make these genes is around 6.6 billion

nucleotides.

Amazingly, most of us have survived this long

without a single, lethal mistake.

But…errors will occur.

An error that causes a change in a DNA/RNA

sequence is called a mutation.

In this chapter, we will talk about 4 types of

mutations

Mutations can take place in gametes, and

affect the offspring, or take place in another

cell in the body and only affect that organism

Mutations can have both positive or negative

results. They can also have no affect

Mutations can be problems with DNA

replication/transcription/translation, or come

from environmental factors

A point mutation is when only one nucleotide

is incorrect.

Even though it is only 1 nucleotide out of 3.3

billion, this one mistake has the potential to

completely change an organism’s health.

Example: “I have a pet cat.” This could be “I

gave a pet cat;” “I wave a pet cat;” “I have a

wet cat.” “I have a pet rat.”

Sickle cell anemia is a disease caused by a

single point mutation that tells the cell to

replace a glutamine with a valine

Frameshift mutations are when at least one

nucleotide is added or deleted from the DNA

or RNA sequence

The correct sequence of amino acids is

dependent on maintaining the 3-nucleotide

codon pattern.

If one nucleotide is added or deleted, the

codon pattern does not start at the right spot

This results in an entire sequence of amino

acids being incorrect.

Chromosome mutations are when the

chromosome itself is damaged before the

DNA is able to be replicated or transcribed

Chromosome mutations tend to occur during

mitosis or meiosis, and usually result in the

death of the cell

It is possible, though rare, for healthy

zygotes to grow with chromosome mutations.

If this happens, the organism will most likely

be sterile

There are four types of chromosome

mutations

1) Deletion. When a section of a chromosome is

deleted

2) Insertion. When part of a chromatid breaks off

and attaches to it’s sister chromatid, resulting in

a duplicate of a chromosome

3) Inversion. When part of a chromosome breaks

but is reattached backwards.

4) Translocation. When part of one chromosome

breaks and attaches to a different chromosome

Mutagens are environmental factors that

cause changes in DNA.

1) Radiation. X-rays, ultraviolet light, cosmic

rays, nuclear radiation all cause radiation by

breaking apart and/or changing DNA

2) Chemicals. Asbestos, benzenes,

formaldehyde react with chemicals in the

DNA

3) High temperatures. High temperatures can

break hydrogen bonds and cause the DNA

strands to fall apart

With all these possibilities, and with 3.3

billion nucleotides, how come mutations

don’t happen more often?

The enzymes that build DNA and RNA strands

have the ability to check for mistakes.

Mistakes happen constantly (probably 1-10

every second). But whenever they do,

enzymes repair the mistake.

Mutations only occur when a mistake occurs

but the enzymes don’t notice OR when an

organism is subjected to multiple mutagens.

This question is worth an extra 5 pts on the biweekly

quiz.

You may check your answers with me ahead of time

for a yes or no response as many times as you like.

Under ideal conditions E. coli can finish

mitosis in only 36 minutes. But it takes 40

minutes to finish replication. How is that

possible?