RNA and Protein Synthesis

Essential Content

A. RNA Synthesis: Transcription (16.3, 16.5)

1. Role in the cell

2. Comparing RNA and DNA

3. Functions of RNA (types)

4. Codons (mRNA; Start and Stop)

B. Protein Synthesis: Translation (16.5)

1. Ribosomes (rRNA)

2. Amino Acids (tRNA; Anticodon)

3. Codon table (both chart & wheel)

4. Translation

5. Gene Expression

C. Types of Mutations: (16.4)

1. Types

a. Point

b. Frameshift

c. Chromosomal

2. Effects

a. Harmful (16.8)

b. Beneficial

c. Variation

d. Neutral

Objectives

· Summarize the process of transcription.

· Identify the different types of RNA (mRNA, tRNA, and rRNA) and explain the structure and function of each

· Demonstrate how specific code sequences are translated into traits through protein synthesis.

· Trace the information flow from DNA to a protein and explain their roles in the expression of genes.

· Describe how amino acids are coded.

· Identify that certain chromosomal mutations can lead to human disorders such as sickle-cell disease.

· Distinguish the cellular processes of DNA replication, transcription, and/or translation

· Students will explain how gene and chromosomal mutations may or may not result in a phenotypic change.

· Relate that mutation and genetic recombination increase genetic variation.

RNA Synthesis

Most of the work of making mRNA takes place during transcription.

In transcription, segments of DNA serve as templates to produce complementary RNA molecules. In prokaryotes (such as bacteria), RNA synthesis and protein synthesis takes place in the cytoplasm. In eukaryotes, RNA is produced in the cell’s nucleus and then moves to the cytoplasm to play a role in the production of protein.

The following focuses on transcription in eukaryotic cells:

The enzyme RNA polymerase binds to DNA during transcription and separates the DNA strands. It then uses one strand of DNA as a template from which to assemble nucleotides into a complementary strand of RNA.

RNA polymerase binds only to promoters, regions of DNA that have specific base sequences. Promoters are signals to the DNA molecule that show RNA polymerase exactly where to begin making RNA. Similar signals cause transcription to stop when a new RNA molecule is completed.

RNA may be “edited” before it is used. Portions that are cut out and discarded are called introns. The remaining pieces, known as exons, are then spliced back together to form the final mRNA.

Types of Mutations Mutations are heritable changes in genetic information. There are two categories of mutations: gene mutations and chromosomal mutations.

Gene mutations produce changes in a single gene. Point mutations involve only one or a few nucleotides. Substitutions, insertions, and deletions are all types of point mutations.

· In a substitution, one base is changed to a different base, which may affect only a single amino acid and have no effect at all.

· In insertions and deletions, one base is inserted or removed from the DNA sequence. Insertions and deletions are called frameshift mutations because they shift the “reading frame” of the genetic message. Frameshift mutations can change every amino acid that follows the point of mutation and can have dramatic effects on the organism.

Chromosomal mutations produce changes in the number or structure of chromosomes. They include deletions, duplications, inversions, and translocations.

· Deletion involves the loss of all or part of a chromosome.

· Duplication produces an extra copy of all or part of a chromosome.

· Inversion reverses the direction of parts of a chromosome.

· Translocation occurs when part of one chromosome breaks off and attaches to another.

Effects of Mutations

Genetic material can be altered by natural events or by artificial means. Errors can be made during replication. Environmental conditions may increase the rate of mutation. Mutagens are chemical or physical agents in the environment that cause mutations.

The effects of mutations on genes vary widely:

Some mutations have little or no effect.

Some mutations produce beneficial variations such as polyploidy in plants (see image below), in which an organism has extra sets of chromosomes. Polyploid plants are often larger and stronger than diploid plants. Mutations can also produce proteins with new or altered functions that can be useful to organisms in different or changing environments.

Some mutations negatively disrupt gene function or dramatically change protein structure.

Genetic disorders such as sickle cell disease can result. (see image of mutation of sickle cell anemia below)