• Today we will be learning about how proteins are made

Protein Synthesis

What do you guys already know about making proteins?

Transcription Introduction Animation

Protein Synthesis: The making of proteins from DNA

DNA mRNA polypeptides

Transcriptio

n Translation

Review Slide: What is DNA Made Of?

• DNA is made of 4 different bases

• They come in pairs• Cytosine and Guanine,

Adenine and Thymine

Who can tell me about

complementary base pairing?

What is RNA Made Of?

• RNA is composed of nucleotides

• RNA is single stranded

• It is made of 4 bases

• Instead of Thymine, RNA has Uracil.

DNA versus RNADNA molecule RNA molecule

Contains a 5-carbon sugar Contains a 5-carbon sugar

5-carbon sugar is deoxyribose

5-carbon sugar is ribose

Each nucleotide has one of four bases

Each nucleotide has one of four bases

Bases are cytosine, guanine, adenine, thymine

Bases are cytosine, guanine, adenine, uracil

Double-stranded molecule Single-stranded molecule

Goal of Transcription

The goal of transcription is to make a mRNA (messenger RNA) strand inside the nucleoplasm that can move into the cytoplasm to be read

Transcription Animation

Transcription Process

1. DNA unzips using the enzyme RNA polymerase

2. The promoter is a sequence of nucleotides that signals for the beginning of the transcription process (this is found on the antisense strand of DNA)

Transcription Process3. RNA nucleotides in the nucleoplasm

complementary base pair with one strand of the DNA molecule

4. The new nucleotides are bonded together by the enzyme RNA polymerase (this period is called elongation)

Important Note: Just like in DNA replication, new nucleotides are added to the 3’ side

5. The transcription process moves along the DNA strand until it hits a terminator sequence

6. The mRNA completely separates and now there is a single strand of mRNA

Note: the mRNA strand is shorter than the DNA strand because it is encoded for one gene and is typically the

length required to make one polypeptide

How does this change for Eukaryotic Cells?

• Note: What we just learned is the simplified version of Transcription for Prokaryotic Cells

• Eukaryotic Cell adjustments:

– Some of the mRNA is non-coding (called introns) and needs to be removed to form mature mRNA (so only the exons are left) before Translation

Comparing Prokaryotic and Eukaryotic Protein Synthesis

DNA Translation

• DNA translation is the process that converts an mRNA sequence into a string of amino acids that form a protein

http://www.dnalc.org/resources/3d/TranslationBasic_withFX0.html

Translation Process

1. mRNA that is now in the cytosplasm will locate a ribosome and align with it so that the start codon can be read

• Codons – a set of three bases in a row on a mRNA molecule; when the codon is read by tRNA (transfer RNA) it codes for a specific amino acid to be made

2. A specific tRNA molecule floats in; the tRNA molecule must have the complementary anticodon to the first mRNA codon

• A tRNA molecule has an anticodon at one end and an amino acid on the other

Amino acid is here!

3. Another tRNA molecule brings in the next set of anticodons to bond with the mRNA codon

4. Now, two amino acids are side by side so an enzyme catalyzes the reaction to bond them together covalently (this makes a peptide bond)

5. The first tRNA molecule can now detach from the amino acid and move out into the cytoplasm to search for another amino acid to join with

6. This process continues until the ribosome reaches the stop codon; this will signal the end of translation

7. The entire polypeptide now breaks away from the final tRNA molecule and becomes a free-floating polypeptide in the cytoplasm

One Gene/One Polypeptide

• In the early 1940’s people hypothesized that for every one gene of DNA produced, one protein was produced– It was later discovered that many proteins are composed of

more than one polypeptide and it was proposed that each individual polypeptide required a separate gene

• In the last few years, researchers have discovered that one gene results in the production of many different polypeptides during translation since mRNA molecules can be modified in different ways

Protein Synthesis Video