Chapter 12DNA TRANSCRIPTION and TRANSLATION

12-3 RNA and Protein Synthesis

WARM UP

What are proteins?Where do they come from?

From DNA to RNA to Protein

DNA in our cells carry the instructions for making proteins in order for our cells to function.

These instructions are found on our genes – sections of DNA that control the production of proteins within the

cell.

How an Organism’s Genotype Determines Its Phenotype

– An organism’s genotype is its genetic makeup, the sequence of nucleotide bases in DNA.

– The phenotype is the organism’s physical traits, which arise from the actions of a wide variety of proteins.

DNA specifies the synthesis of proteins in two stages:1. transcription, the transfer of genetic information

from DNA into an RNA molecule

2. translation, the transfer of information from RNA into a protein.

Central Dogma of Biology

Central Dogma

Polypeptide

TRANSCRIPTION

mRNA

DNA

Gene

TRANSLATION

© 2013 Pearson Education, Inc.

Bioflix Animation: Protein Synthesis

Figure 10.8-1

DNA

Cytoplasm

Nucleus

Figure 10.8-2

DNA

Cytoplasm

NucleusRNA

TRANSCRIPTION

Figure 10.8-3

DNA

Cytoplasm

NucleusRNA

Protein

TRANSCRIPTION

TRANSLATION

• RNA = ribonucleic acid (ribose sugar)

• Instead of THYMINE, RNA has URACIL

• Single strand

RNA Overview

– mRNA = messanger RNA

– tRNA = transfer RNA

– rRNA = ribosomal RNA

3 Types of RNA

– mRNA = messanger RNA• made by transcription of the original DNA

molecule

• a messanger from the DNA to the rest of the cell.

3 Types of RNA

– tRNA = transfer RNA• interpreter, converts the language in mRNA into

the language of proteins (amino acid monomers)

3 Types of RNA

– rRNA = ribosomal RNA• rRNA is combined with proteins to form a

ribosome, which is the site of protein synthesis

• coordinate the functions of mRNA and tRNA

3 Types of RNA

Figure 10.10a

Amino acid

RNA

DNA strand

Polypeptide

Codon

TRANSCRIPTION

TRANSLATION

DNA to mRNA Problems

Given the DNA sequence, construct an mRNA for the following pieces of DNA.

TTCAGCGATACCGTAGGA

TACCCCGTATTGGAAATT

AAACCGGCAAAATTGCTC

The role of a master plan in a building is similar to the role of which molecule?

a. messenger RNA

b. DNA

c. transfer RNA

d. ribosomal RNA

The nucleic acid responsible for bringing individual amino acids to the ribosome is

a. transfer RNA.

b. DNA.

c. messenger RNA.

d. ribosomal RNA.

Transcription: From DNA to RNA

Transcription

• makes mRNA from a DNA template,

• substitutes uracil (U) for thymine (T).

• uses a process that resembles the synthesis of a DNA strand during DNA replication

Lets take a closer look at this process!!!

Transcription: From DNA to RNA

THE BIG PICTURE

– Transfer of genetic information from DNA to RNA resembles the process of DNA replication!

– Only 1 strand of DNA is used as a template for mRNA synthesis.

– RNA nucleotides are linked by the transcription enzyme RNA polymerase.

– End result is mRNA!!!

Figure 10.13a

Newly

made

RNA

RNA nucleotides

Template

strand of DNA

(a) A close-up view of transcription

RNA

polymerase

Transcription

– The “start transcribing” signal is a nucleotide sequence on DNA called a promoter• located in the DNA sequence at the beginning of

the gene which is being transcribed.

• a specific place where RNA polymerase attaches.

Figure 10.13b

(b) Transcription of a gene

RNA polymerase

Completed RNA

Growing RNATermination

Initiation Terminator DNA

ElongationRNA

Promoter

DNA

RNA polymerase

DNA of gene

2

1

3

The Processing of Eukaryotic RNA

So…we now have an RNA strand….now what?

– Prokaryotic cells• RNA transcribed from a gene immediately

functions as messenger RNA (mRNA), the molecule that is translated into protein.

– Eukaryotic cells• localizes transcription in the nucleus (because they

have one!!)

• modifies, or processes, the RNA transcript in the nucleus before it move to the cytoplasm for translation by a ribosome.

– RNA processing includes• removing introns (noncoding regions of the RNA)

• RNA splicing, joining exons (the parts of the gene that are expressed) together to form messenger RNA (mRNA).

The Processing of Eukaryotic RNA

A region of a DNA molecule that indicates to an enzyme where to bind to make RNA is the

a. intron.

b. exon.

c. promoter.

d. codon.

The Genetic Code

How does a piece of mRNA now make a protein?

The genetic code is the “language” of mRNA instructions that are translated by rRNA into the language of proteins.

The code is written using four “letters”

• A (adenine)

• U (uracil)

• C (cytosine)

• G (guanine)

The Genetic Code

• Proteins are made by joining amino acidstogether in long chains called polypeptides (like beads on a string!!)

• Each polypeptide contains a combination of any or all of the 20 different amino acids.

• The property of each protein is determined by the order in which the amino acids are joined together.

The Genetic Code

The Genetic Code

• An mRNA strand is read three nucleotides at a time.

• A three nucleotide sequence is called a codon.

• Each codon specifies a particular amino acid (monomer of protein)

The Genetic Code

mRNA sequence

UCGCACGGU

will be read 3 letters at a time

UCG-CAC-GGU

Each codon (three letter sequence) represents and amino acid.

UCG – CAC- GGUSerine-Histidine-Glycine

The Genetic Code

A codon typically carries sufficient information to specify a(an)

a. single base pair in RNA.

b. single amino acid.

c. entire protein.

d. single base pair in DNA.

Translation

• 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 (mRNA) to produce proteins.

• tRNA carry specific amino acids based on the mRNAmessage.

Translation

• Each tRNA molecule only carries 1 amino acid.

• tRNA also has 3 unpaired bases called anticodons, which are complementary to 1 mRNA codon.

Steps of Translation

1. mRNA leaves the nucleus via nuclear pore.2. mRNA attaches to a ribosome3. The ribosome reads the first codon (3 letter sequence)4. tRNA brings the proper amino acid into the ribosome5. The ribosome moves down the mRNA and reads the 2nd codon6. tRNA brings the proper amino acid into the ribosome7. The ribosome forms a bond between the first and second

amino acid.8. The first tRNA is released from the ribosome.9. The ribosome continues down the mRNA reading each codon

and tRNA brings in the amino acid and the polypeptide chain grows.

10.The polypeptide chain grown until the ribosome reaches a STOP codon on the mRNA molecule, at which point it releases the newly formed protein and mRNA.