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Lesson Overview Fermentation

Lesson Overview 13.1 RNA


THINK ABOUT IT

DNA is the genetic material of cells. The

sequence of nucleotide bases in the

strands of DNA carries some sort of

code. In order for that code to work, the

cell must be able to understand it.

What, exactly, do those bases code for?

Where is the cell’s decoding system?

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The Role of RNA

How does RNA differ from DNA?


The Role of RNA

How does RNA differ from DNA?

There are three important differences

between RNA and DNA: (1) the sugar in

RNA is ribose instead of deoxyribose, (2)

RNA is generally single-stranded and not

double-stranded, and (3) RNA contains

uracil in place of thymine.

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The Role of RNA

Genes contain coded DNA instructions

that tell cells how to build proteins.

The first step in decoding these genetic

instructions is to copy part of the base

sequence from DNA into RNA.


The Role of RNA

RNA, like DNA, is a nucleic acid

that consists of a long chain of

nucleotides.

RNA then uses the base sequence

copied from DNA to direct the

production of proteins.

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Comparing RNA and DNA

Each nucleotide in both DNA and

RNA is made up of a 5-carbon sugar,

a phosphate group, and a

nitrogenous base.

There are three important differences

between RNA and DNA:



(1) The sugar in RNA is ribose instead of

deoxyribose.

(2) RNA is generally single-stranded and not

double-stranded.

(3) RNA contains uracil in place of thymine.

These chemical differences make it easy for the

enzymes in the cell to tell DNA and RNA apart.

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The roles played by DNA and RNA are

similar to the master plans and blueprints

used by builders.



A master plan has all the information needed to

construct a building. Builders never bring a

valuable master plan to the building site, where

it might be damaged or lost. Instead, they

prepare inexpensive, disposable copies of the

master plan called blueprints.

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Similarly, the cell uses DNA “master

plan” to prepare RNA “blueprints.”

The DNA molecule stays safely in the

cell’s nucleus, while RNA molecules

go to the protein-building sites in the

cytoplasm—the ribosomes.


Functions of RNA

You can think of an RNA molecule, as a

disposable copy of a segment of DNA, a working

copy of a single gene.

RNA has many functions, but most RNA

molecules are involved in protein synthesis only.

RNA controls the assembly of amino acids into

proteins. Each type of RNA molecule specializes

in a different aspect of this job.

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Functions of RNA

The three main types of RNA

are messenger RNA, ribosomal

RNA, and transfer RNA.

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Messenger RNA

Most genes contain

instructions for assembling

amino acids into proteins.

The RNA molecules that carry

copies of these instructions

are known as messenger

RNA (mRNA): They carry

information from DNA to other

parts of the cell.


Ribosomal RNA

Proteins are assembled on

ribosomes, small

organelles composed of

two subunits.

These ribosome subunits

are made up of several

ribosomal RNA (rRNA)

molecules and as many as

80 different proteins.

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Transfer RNA

When a protein is built,

a transfer RNA (tRNA)

molecule transfers

each amino acid to the

ribosome as it is

specified by the coded

messages in mRNA.


RNA Synthesis

How does the cell make RNA?

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RNA Synthesis

How does the cell make RNA?

In transcription, segments of

DNA serve as templates to

produce complementary RNA

molecules.


Transcription

Most of the work of making RNA takes

place during transcription. During

transcription, segments of DNA serve as

templates to produce complementary RNA

molecules.

The base sequences of the transcribed

RNA complement the base sequences of

the template DNA.

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Transcription

• In prokaryotes, RNA synthesis and protein

synthesis take place in the cytoplasm.

• In eukaryotes, RNA is produced in the

cell’s nucleus and then moves to the

cytoplasm to play a role in the production

of proteins. Our focus will be on

transcription in eukaryotic cells.


Transcription

Transcription requires an enzyme, known as

RNA polymerase, that is similar to DNA

polymerase.

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Transcription

RNA polymerase binds to DNA during

transcription and separates the DNA strands.


Transcription

RNA polymerase then uses one strand of DNA as a

template from which to assemble nucleotides into a

complementary strand of RNA.

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Promoters

RNA polymerase binds only to promoters,

regions of DNA that have specific base

sequences.

Promoters are signals in the DNA molecule that

show RNA polymerase exactly where to begin

making RNA.

Similar signals in DNA cause transcription to

stop when a new RNA molecule is completed.

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RNA Editing

RNA molecules

sometimes require bits

and pieces to be cut out

of them before they can

go into action.

The portions that are cut

out and discarded are

called introns.


RNA Editing

In eukaryotes, introns

are taken out of pre-

mRNA molecules while

they are still in the

nucleus.

The remaining pieces,

known as exons, are

then spliced back

together to form the final

mRNA.

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RNA Editing

Biologists don’t have a complete

answer as to why cells use

energy to make a large RNA

molecule and then throw parts of

that molecule away.

Some pre-mRNA molecules may

be cut and spliced in different

ways in different tissues, making

it possible for a single gene to

produce several different forms of

RNA.

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RNA Editing

Introns and exons may

also play a role in

evolution, making it

possible for very small

changes in DNA

sequences to have

dramatic effects on how

genes affect cellular

function.


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Lesson Overview 13.2 Ribosomes and

Protein Synthesis


THINK ABOUT IT

How would you build a system to read the messages

that are coded in genes and transcribed into RNA?

Would you read the bases one at a time, as if the

code were a language with just four words—one

word per base?

Perhaps you would read them as individual letters

that can be combined to spell longer words.

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The Genetic Code

• What is the genetic code, and how is it

read?


The Genetic Code

What is the genetic code, and how is it read?

The genetic code is read three

“letters” at a time, so that each

“word” is three bases long and

corresponds to a single amino

acid.

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The Genetic Code

The first step in decoding genetic

messages is to transcribe a nucleotide

base sequence from DNA to RNA.

This transcribed information contains a

code for making proteins.


The Genetic Code

Proteins are made by joining amino acids together

into long chains, called polypeptides.

As many as 20 different amino acids are

commonly found in polypeptides.

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The Genetic Code

The specific amino acids in a polypeptide,

and the order in which they are joined,

determine the properties of different

proteins.

The sequence of amino acids influences the

shape of the protein, which in turn

determines its function.


The Genetic Code

RNA contains four different bases:

adenine, cytosine, guanine, and

uracil.

These bases form a “language,” or

genetic code, with just four

“letters”: A, C, G, and U.

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The Genetic Code

Each three-letter “word” in mRNA is known as a

codon.

A codon consists of three consecutive bases that

specify a single amino acid to be added to the

polypeptide chain.


How to Read Codons

Because there are four

different bases in RNA, there

are 64 possible three-base

codons (4 × 4 × 4 = 64) in

the genetic code.

This circular table shows the

amino acid to which each of

the 64 codons corresponds. To

read a codon, start at the

middle of the circle and move

outward.

pg. 367

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How to Read Codons

Most amino acids can be

specified by more than one

codon.

For example, six different

codons—UUA, UUG, CUU,

CUC, CUA, and CUG—

specify leucine. But only

one codon—UGG—

specifies the amino acid

tryptophan.

pg. 367


Start and Stop Codons

The genetic code has punctuation

marks.

The methionine codon AUG

serves as the initiation, or “start,”

codon for protein synthesis.

Following the start codon, mRNA

is read, three bases at a time, until

it reaches one of three different

“stop” codons, which end

translation.

pg. 367

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pg. 367


Translation

What role does the

ribosome play in

assembling proteins?

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Translation

What role does the ribosome play in assembling proteins?

Ribosomes use the sequence of

codons in mRNA to assemble

amino acids into polypeptide

chains.


Steps in Translation

Messenger RNA is transcribed in the nucleus

and then enters the cytoplasm for translation.

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Translation begins when a

ribosome attaches to an

mRNA molecule in the

cytoplasm.

As the ribosome reads each

codon of mRNA, it directs

tRNA to bring the specified

amino acid into the ribosome.

One at a time, the ribosome

then attaches each amino acid

to the growing chain.


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Bell Ringer: • Use the codon chart on page 367 to

determine what the following amino acid sequence would be for the mRNA message below:

AUGCUCGAGAAC

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Bell Ringer:

• Met – Leu – Glu. acid – Asp.

AUGCUCGAGAAC


Translation

• The sequence of nucleotide bases in an

mRNA molecule is a set of instructions that

gives the order in which amino acids should

be joined to produce a polypeptide.

• The forming of a protein requires the folding

of one or more polypeptide chains.

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Translation

• Ribosomes use the sequence of

codons in mRNA to assemble amino

acids into polypeptide chains.

• The decoding of an mRNA message

into a protein is a process known as

translation.



Messenger RNA is transcribed in the nucleus and then enters the

cytoplasm for translation.

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Translation begins when a

ribosome attaches to an

mRNA molecule in the

cytoplasm.

As the ribosome reads each

codon of mRNA, it directs

tRNA to bring the specified

amino acid into the ribosome.

One at a time, the ribosome

then attaches each amino acid

to the growing chain.



Each tRNA molecule carries just

one kind of amino acid.

In addition, each tRNA molecule

has three unpaired bases,

collectively called the

anticodon—which is

complementary to one mRNA

codon.

The tRNA molecule for

methionine has the anticodon

UAC, which pairs with the

methionine codon, AUG.

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The ribosome has a

second binding site for a

tRNA molecule for the next

codon.

If that next codon is UUC,

a tRNA molecule with an

AAG anticodon brings the

amino acid phenylalanine

into the ribosome.



The ribosome helps form

a peptide bond between

the first and second amino

acids—methionine and

phenylalanine.

At the same time, the

bond holding the first

tRNA molecule to its

amino acid is broken.

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That tRNA then moves

into a third binding

site, from which it exits

the ribosome.

The ribosome then

moves to the third

codon, where tRNA

brings it the amino

acid specified by the

third codon.



The polypeptide chain

continues to grow until

the ribosome reaches a

“stop” codon on the

mRNA molecule.

When the ribosome

reaches a stop codon, it

releases both the newly

formed polypeptide and

the mRNA molecule,

completing the process

of translation.

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The Roles of tRNA and rRNA in

Translation

Ribosomes are composed of roughly 80 proteins

and three or four different rRNA molecules.

These rRNA molecules help hold ribosomal

proteins in place and help locate the beginning

of the mRNA message.

They may even carry out the chemical reaction

that joins amino acids together.


The Molecular Basis of Heredity

What is the “central dogma” of molecular

biology?

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What is the “central dogma” of molecular biology?

The central dogma of molecular

biology is that information is

transferred from DNA to RNA to

protein.



Most genes contain instructions for assembling

proteins.

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Many proteins are enzymes, which catalyze and regulate

chemical reactions.

A gene that codes for an enzyme to produce pigment can

control the color of a flower. Another gene produces

proteins that regulate patterns of tissue growth in a leaf.

Yet another may trigger the female or male pattern of

development in an embryo.

Proteins are microscopic tools, each specifically designed

to build or operate a component of a living cell.



Molecular biology seeks to explain living organisms by studying them at the

molecular level, using molecules like DNA and RNA.

The central dogma of molecular biology is that information is transferred

from DNA to RNA to protein.

There are many exceptions to this “dogma,” but it serves as a useful

generalization that helps explain how genes work.

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Gene expression is the way in which

DNA, RNA, and proteins are involved in

putting genetic information into action in

living cells.



Proteins play a key role in producing an

organism’s traits.

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One of the most interesting discoveries of molecular biology is the near-

universal nature of the genetic code.

Although some organisms show slight variations in the amino acids

assigned to particular codons, the code is always read three bases at a

time and in the same direction.

Despite their enormous diversity in form and function, living organisms

display remarkable unity at life’s most basic level, the molecular biology of

the gene.

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