13.3 RNA and Gene Expression

Mrs. Stipanovich

How do genes (DNA) code for proteins?

DNA contains the instructions or code for the production of proteins in the cell.

This is known as gene expression. Proteins determine the structure

and function of organisms (traits).

How do genes (DNA) code for proteins? Two processes:

Transcription: DNA ----- RNA Translation: RNA ------ Protein

Proteins are made on the ribosomes that are located in the cytoplasm of the cell. DNA in the nucleus and is too large to leave the nucleus and travel to the cytoplasm.

How can the genetic code get to the ribosomes?

RNA- Ribonucleic Acid

Similar to DNA with three differences:1. Made up of one strand2. Contains the base Uracil (U) instead of Thymine.3. Contains the sugar ribose instead of deoxyribose.

Three Types of RNA

1. Messenger (mRNA) – carries copies of the instructions for making proteins from the DNA (in the nucleus) to the ribosomes in the cytoplasm of the cell.

2. Ribosomal (rRNA) – helps to assemble amino acids to make proteins on the ribosomes.

3. Transfer (tRNA) – transfers specific amino acids to the ribosome for assembly into proteins

Transcription: Reading the gene

Transcription is the copying (transcribing) of a certain gene on DNA onto RNA.

Steps of Transcription

1. The DNA strand has a code for “start,” the RNA Polymerase is triggered to start reading the code at this spot.

2. RNA Polymerase unzips the DNA to make a copy of one section of it.

3. The RNA nucleotides line up to make a copy of the DNA.

Transcription

RNA

Codons

Every word in DNA “language” is a three letter word known as a codon.

Each codon is code for a certain amino acid.

When several amino acids are put together you get a protein.

There are twenty different amino acids that make up all proteins.

Proteins control your traits.

Translation: RNA to Proteins

Translation is essentially decoding the codons to know which amino acids the cell should make.

Steps of Translation

1. mRNA attaches to a ribosome

Steps of Translation

2. Each mRNA codon moves along the ribosome and the tRNA anticodon brings the correct amino acid to the ribosome.

Steps of Translation

3. Peptide bonds form between the amino acids and the tRNA molecule is released.

Steps of Translation

4. The process continues until a stop codon is reached. There are 3 stop codons: UAA, UAG, UGA

Now you try!

Use your codon wheel to translate the following codons into amino acids.

AUG UGU GGG