chapter 14 translation

The _____ _____ is the covalent bond between amino acids in a _______

peptide bond ; polypeptide

where does the peptide bond form between polypeptides?

It forms between the carboxyl end of an amino acid and the amino end of the next.

Many proteins consist of a

single polypeptide

Calmodulin is present in all _______ cells.

eukaryotic

what is calmodulin

a small protein that consists of a single polypeptide

what structure does calmodulin have

teritiary structure

calmodulin has _____ helices

alpha helices, a type of secondary structure observed in many proteins.

hemoglobin is an example of what

a protein with multiple polypeptides

Hemoglobin (Hb) is

the protein inside red blood cells that carries oxygen in the bloodstream

what is hemoglobulin made of

The complete protein has four polypeptides, two α-globins and two β-globins, and therefore has a quaternary structure.

Polypeptide:

a sequence of amino acids (1ry structure)

Alpha helix:

partial folding (2ry structure)

Globin:

complete folded polypeptide (3ry structure)

Hemoglobin:

four globins (4ry structure)

In sickle-cell anemia, a single amino acid substitution in

_______ reduces the affinity of Hb for ___ and impairing

the solubility and absorption of oxygen. 

β-globin ; O₂

in sickle cell anemia, The polypeptide forms large polymers that

distort the shape of red blood cells, affecting blood flow and oxygen transport.

Codon:

a three-base (nucleotides) sequence of DNA (a triplet) that codes for an amino acid.

start codon:

AUG, which also codes for methionine

stop codons:

UAA, UAG, and UGA (these do not code for

any amino acids)

the genetic code is expressed as

mRNAs in the 5’ => 3’ orientation

Characteristics of the genetic code

Unambiguous:

each of the 61 triplets code for only one of

the 20 amino acids.

Characteristics of the genetic code

Degenerate:

most amino acids are encoded by more than

one codon.

Characteristics of the genetic code

Universal:

most living organisms use the same code, but 

exceptions exist.

Characteristics of the genetic code

Commaless:

there are no breaks between the codons in a reading frame (all the bases of the translated sequences are part of codons).

Characteristics of the genetic code

Non-overlapping:

the triplets in a reading frame are in a tandem sequence and do not overlap.

Spontaneous mutations can also result in ______ changes

permanent

Spontaneous mutations are or are not caused by errors in DNA replication?

are not

DNA exists inside of the cell, which is an ___ system. 

open

DNA is exposed to _____ and _____ that can react with it and cause chemical alterations.

radiations and molecules

When a base is ______ by a different base in DNA, the result is a permanent ____ ____ ____

replaced ; single codon change.

Silent:

the resulting new codon codes for the same amino acid as the original codon.

Missense:

the new codon codes for a different amino acid.

Nonsense:

the new codon is a stop codon, resulting in a shorter polypeptide.

mutations may also be caused by

errors in DNA replication

mutations occur in the ___ and may be copied to ____

DNA ; mRNAs

what are also due to errors in DNA replication

insertions

All the codons from the insertion site on are changed, resulting in a 

frameshift

The _____ of single bases by errors in DNA replication also occur and also result in frameshift mutations.

deletions

what type of disease is Huntington's disease

neurodegenerative

describe huntingtons disease

• It is an autosomal dominant disease

• caused by the expansion of trinucleotide CAG repeats, which code for glutamine in the mRNA, in the huntintin gene resulting in polyglutamine Huntingtin proteins.

Basic requirements of translation

• mRNA

• charged transfer RNAs (tRNAs)

• ribosome

• other requirements: initiation factors, elongation factors, and energy sources

• no primers of any kind

An amino acid is attached to the

3’ end of the tRNA.

The ______ loop base-pairs with a codon in an ______.

anticodon ; mRNA

Inosine is a

modified adenine.

The post-transcriptional modification of bases in tRNAs result in

unusual bases.

what is the wobble hypothesis

• The interaction between the third position of the codon in the mRNA and the first position of the anticodon in the tRNA is less critical and less constrained.

     •some tRNA bases can pair with multiple mRNA bases at        this position

    •this allows translation to occur without the need for the cell to synthesize all 61 tRNAs

    •inosine is a post-transcriptionally modified adenine that can occur at the wobble site of tRNAs

Charging:

the carboxyl group of the amino acid is covalently attached to the 3’ end of a tRNA.

Charging is catalyzed by twenty different Aminoacyl tRNA

twenty different Aminoacyl tRNA synthases

Energy is spent in charging in order to eventually make a

new ____ ____ in the ribosome.

peptide bond

charging step 1

amino acid activation:

• the amino acid (AA) is converted to an aminoacyladenylic acid (AA-AMP).

• This is the energy-consuming step in the process of

eventually making a new peptide bond. It requires ATP

charging step 2

• the aminoacyladenylic acid loses the AMP and the carboxyl group of the amino acid is attached to the 3’ end of a tRNA.

• The result is an aminoacyl tRNA (a charged tRNA).

Ribosome:

• large particle of rRNA and proteins where translation occurs

• two subunits: small and large

The Svedberg unit (S) is not a measure of molecular weight,

but a measure of

the rate at which particles sediment in a

centrifugal field.

the rate of svedberg unit depends on

weight, shape, and size. 

Svedberg values

(for prokaryotic ribosomes):

•small subunit: 30S

•large subunit: 50S

•complete ribosome: 70S

Steps in the initiation of translation:

1. mRNA binds to the small ribosomal subunit with

 the AUG codon positioned on the P site.

2. f-Met-tRNA (in prokaryotes) binds to the AUG codon.

3. The large ribosomal subunit joins the complex.

This process requires GTP for a source of energy plus a

series of initiation factors (IF proteins).

EF-Tu (an elongation factor) and GTP facilitate the binding

of the ____ _____ to the second codon at the __ site

second tRNA ; A

the amino acid on the first tRNA is transferred and forms

a _____ ____ with the amino acid on the second tRNA

(a _____ forms); the first tRNA moves to the E site

peptide bond ; dipeptide

the mRNA is shifted to place the second codon in the __ site

and to bring the third codon into the __ site

P ; A

the tRNA carrying the third amino acid binds the third codon on the _ site

A

the dipeptide on the second tRNA is transferred to form a 

_____ attached to the third tRNA; the second tRNA

moves to the E site with the help of EF-G and GTP

tripeptide

Termination of translation

step 1:

A STOP codon moves to the A site of the ribosome. No tRNAs bind to stop codons.

Termination of translation

step 2:

Release factor 1 (RF₁) binds to the stop codon

Termination of translation

step 3:

the GTP-dependent Release factor 3 (RF₃) releases the

polypeptide chain from the last tRNA and the entire

translation complex dissociates 

The diphtheria toxin inhibits

the last step of the polypeptide elongation cycle

The diphtheria toxin is produced by

Corynebacterium diphtheriae.

• inhibits the eukaryotic elongation

  factor 2 (EF2), which is analogous to the prokaryotic EF-G.

  Translation is blocked and no new proteins can be made.