Essential Idea Genetic information in DNA can be accurately copied and can be translated to make the...

Essential Idea Genetic information in DNA can be

accurately copied and can be translated to make the proteins needed by the cell.

Understandings Transcription is the synthesis of mRNA copied from the

DNA base sequences by RNA polymerase. Translation is the synthesis of polypeptides on ribosomes. The amino acid sequence of polypeptides is determined by

mRNA according to the genetic code. Codons of three bases on mRNA correspond to one amino

acid in a polypeptide. Translation depends on complementary base pairing

between codons on mRNA and anticodons on tRNA.

Skills and Applications Skill: Use a table of the genetic code to deduce which

codon(s) corresponds to which amino acid. Skill: Use a table of mRNA codons and their

corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.

Skill: Deducing the DNA base sequence for the mRNA strand.

 RNA and Protein SynthesisDNA, RNA, Protein Synthesis & GENES

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IB Assessment Statement

Compare the structure of RNA and DNA.

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DNA, RNA, PROTEIN SYNTHESIS & GENES

Genes are coded DNA instructions that control the production of proteins.

Genetic messages can be decoded by copying part of the nucleotide sequence from DNA into RNA.

RNA contains coded information for making proteins.

The Structure of RNA

The Structure of RNA

RNA consists of a long chain of nucleotides.

Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.

The Structure of RNA

There are three main differences between RNA and DNA:

1. The sugar in RNA is ribose instead of deoxyribose.

2. RNA is generally single-stranded.

3. RNA contains uracil in place of thymine.

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DNA vs. RNA

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Types of RNA

Types of RNA

There are three main types of RNA:

1. messenger RNA

2. ribosomal RNA

3. transfer RNA

Types of RNA

Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins.

Types of RNA

Ribosomes are made up of proteins and ribosomal RNA (rRNA).

Ribosome

Ribosomal RNA

Types of RNA

During protein construction, transfer RNA (tRNA) transfers each amino acid to the ribosome.

Amino acid

Transfer RNA

3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

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Transcription

Transcription

RNA molecules are produced by copying part of a nucleotide sequence of DNA into a complementary sequence in RNA. This process is called transcription.

Transcription requires the enzyme RNA polymerase.

Transcription

During transcription, RNA polymerase binds to DNA and separates the DNA strands.

RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.

Practice Base Pairing of DNA to mRNA

DNA Template Strand

Complementary RNA Strand

Adenine A

Thymine T

Guanine G

Cytosine C

A

A

G

T

C

Uracil U

Adenine

Cytosine

Guanine

U

G

CA

U

Transcription Summary

• During Transcription, only one strand of DNA double helix serves as a template for the synthesis of mRNA.

• The strand of DNA that is the template for mRNA is called the antisense.

• The strand of DNA that is NOT a template for mRNA is called the sense strand.

• RNA polymerase binds only to regions of DNA known as promoters.

• Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.

This model illustrate the process of transcription that takes place in the nucleus. The DNA base sequence of the gene is copied into messenger RNA (mRNA)

1. The DNA helix is opened at the position of the gene.

2. The helix is unwound by RNA polymerase

3. RNA nucleotides are found in the nucleus space.

4. One of the DNA chains act as a template for mRNA

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This model illustrate the process of transcription that takes place in the nucleus. The DNA base sequence of the gene is copied into messenger RNA (mRNA)

5. Free nucleotides base pair with DNA nucleotides6. The phosphodiester bonds on the mRNA chain are

formed by RNA polymerase7. mRNA is a single polynucleotide chain but the base

thymine is replaced by Uracil.

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This model illustrate the process of transcription that takes place in the nucleus. The DNA base sequence of the gene is copied into messenger RNA (mRNA)

8 After the mRNA is complete the molecule detach's from the DNA and leaves the nucleus for the cytoplasm ribosomes.

9 The DNA helix reforms.

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Transcription

RNA

RNA polymerase

DNA

IB Assessment Statement

Describe the genetic code in terms of codons composed of triplets of bases.

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The Genetic Code

The Genetic Code

The genetic code is the “language” of mRNA instructions.

The code is written using four “letters” (the bases: A, U, C, and G).

The Genetic Code

A codon consists of three consecutive nucleotides on mRNA that specify a particular amino acid.

The Genetic Code

Each codon specifies a particular amino acid that is to be placed on the polypeptide chain.

Some amino acids can be specified by more than one codon.

The Genetic Code

The Genetic Code

There is one codon AUG that can either specify the amino acid methionine or serve as a “start” codon for protein synthesis.

There are three “stop” codons that do not code for any amino acid. These “stop” codons signify the end of a polypeptide.

Practicing Translation from mRNA to Amino Acid

mRNA Amino Acid

AUG

CCC

AGG

GGA

GCA

AGC

AGU

Methionine

Proline

Arginine

Glycine

Alanine

Serine

Serine

More on the genetic code:

The genetic code:

• A polynucleotide is a sequence of bases

• Bases are either A T G or C

• There are 4 bases which operate in sets of 3 (a triplet).= 43possible triplets of DNA =64 triplets

• There are 20 common amino acids

• Therefore 64 triplets are mapped to 20 amino acids

• However there is a 'punctuation' triplets.

• Therefore the mapping of the code is 64: 21

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• The genetic code is first transcribed into mRNA

• The mRNA codons can be mapped to a specific amino acid.The mapping is 64 triplets: 64 codons: 21

• DNA is a degenerate code since there are more than one triplet or codon that maps to an amino acid or punctuation.

• mRNA codon AUG codes for Methionine and is a START signal for translation.

• mRNA codon UAA, UAG, UGA are all stop codons punctuating the code.

• GGU, GGC, GGA and GGG all code for amino acid glycine.

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IB Assessment Statement:

Explain the process of translation, leading to polypeptide formation.

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TranslationTranslation

Translation is the decoding of an mRNA message into a polypeptide chain (protein).

Translation takes place on ribosomes.

During translation, the cell uses information from messenger RNA to produce proteins.

Translation

Nucleus

mRNA

Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.

Translation

Translation begins when an mRNA molecule attaches to a ribosome.

As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA.

In the ribosome, the amino acid is transferred to the growing polypeptide chain from the cytoplasm.

Translation

Each tRNA molecule carries only one kind of amino acid.

In addition to an amino acid, each tRNA molecule has three unpaired bases.

These bases, called the anticodon, are complementary to one mRNA codon.

The Structure and Function of Transfer RNA

ACC

A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long

Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf

LE 17-14a

Amino acidattachment site

Hydrogenbonds

3

5

Two-dimensional structureAnticodon

Amino acidattachment site

35

Hydrogenbonds

Anticodon Anticodon

Symbol used in this bookThree-dimensional structure

3 5

Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional moleculetRNA is roughly L-shaped

Translation

LysinetRNAPhenylalanine

Methionine

Ribosome

mRNAStart codon

The ribosome binds new tRNA molecules and amino acids as it moves along the mRNA.

Ribosomes

Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis

The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)

tRNA

Ribosome

mRNA

Lysine

Translation direction

33’’55’’

Protein Synthesis (building a polypeptide):

• Ribosomes are the site of polypeptides synthesis.

• This involves linking amino acids together through condensation (dehydration) reactions.

tRNA

Ribosome mRNA

Lysine

Translation direction33’’55’’

Elongation of the Polypeptide Chain

At the ribosome, amino acids are added one by one to the preceding amino acid

A peptide bond is formed between the amino acids.

Translation

•The process continues until the ribosome reaches a stop codon.

Polypeptide

Ribosome

tRNA

mRNA

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Genes and Proteins

The sequence of bases in DNA is used as a template for mRNA.

The codons of mRNA specify the sequence of amino acids in a protein.

CodonCodon Codon

Codon Codon Codon

mRNA

Alanine Arginine Leucine

Amino acids within a polypeptide

Single strand of DNA

http://www.wiley.com/legacy/college/boyer/0470003790/animations/translation/translation.htm

https://www.youtube.com/watch?v=itsb2SqR-R0

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IB Assessment Statement

Discuss the relationship between one gene and one polypeptide. Originally, it was assumed that one gene would invariably code for one polypeptide, but many exceptions have been discovered.

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Genes and Proteins

Genes and Proteins

Genes contain instructions for assembling proteins.

Many proteins are enzymes, which catalyze and regulate chemical reactions.

Proteins are each specifically designed to build or operate a component of a living cell.

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One Gene One Polypeptide:

Theory: One gene is transcribed and translated to produce one polypeptide.

Some proteins are composed of a number of polypeptides and in this theory each polypeptide has its own gene.

e.g. haemoglobin is composed of 4 polypeptides (2 of each type) and there is a gene for each type of polypeptide.

This theory, like so many in biology has exceptions. e.g.

1) Some genes code for types of RNA which do not produce polypeptides.

2) Some genes control the expression of other genes.

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Animation on one gene one polypeptide:

http://www.youtube.com/watch?v=Thj6jq7mYkE

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