Let’s Review!
Where is the DNA stored?
In the nucleus
What is the membrane around that organelle called?
Nuclear envelope
What are the openings in that membrane that lead to other parts of the cell called?
Nuclear pores
What organelle makes proteins?
Ribosomes
While DNA has the information for all the cell’s activities, it is not directly involved in the day to day
operations of the cell.
Proteins are responsible for implementing the instructions contained in DNA
Each gene along a DNA molecule directs the synthesis of a specific type of messenger RNA molecule
(mRNA).
The mRNA interacts with the protein-synthesizing machinery to direct the ordering of amino acids in a
polypeptide.
RNA= ribonucleic acid
Ribonucleic acid
Ribose=
Single stranded
Needed for the production of proteins
Nitrogenous base uracil instead of thymine
Steps from DNA to Proteins
1. DNA is copied or transcribed into a single strand of mRNA (messenger RNA)
2. The mRNA exits the nucleus through a nuclear pore
3. The mRNA then is read by Ribosomal RNA.
4. In the ribosome, the mRNA is read and amino acids attached to tRNA (transfer RNA) are
assembled to make proteins.
DNA -> RNA -> Protein
Genetic code
DNA contains a triplet code
Every three bases on a DNA strand code for one amino acid
Each three-letter unit on mRNA is called a codon
Some amino acids can have more than one codon
The code is nearly universal to all living organisms
All animals, plants, fungi, bacteria, archaea, and viruses use this same genetic code.
All these organisms evolved from the same chemical basis, so the same chemical process continued
through evolution.
Transcription
Transcription= process in which DNA’s nucleotide sequence is converted to the form of a singlestranded RNA molecule
mRNA= messenger RNA
RNA molecule transcribed from a DNA template
DNA molecule unzips
RNA bases pair with the complementary DNA bases
RNA has uracil instead of thymine
RNA polymerases links the RNA nucleotides together
RNA polymerase

Enzyme found in the nucleus

Separates the two DNA strands by breaking the hydrogen bonds between the bases

Then moves along one of the DNA strands and links RNA nucleotides together
In prokaryotes, the mRNA transcribed from a gene is the direct messenger molecule needed to make a
protein
In eukaryotes, the mRNA has to be modified before it leaves the nucleus
Noncoding nucleotides interrupt the nucleotide sequences
Internal noncoding sequences are called introns
Researchers are still trying to determine their function
The coding regions, or the parts of the gene that will be expressed are called exons
Before mRNA leaves the nucleus, the introns are removed and the exons are connected together in a
process called RNA splicing
Translation
tRNA= transfer RNA, translates the three-letter codons of mRNA to the amino acids that make up
proteins (an “interpreter”)
Picks up the appropriate amino acid floating in the cytoplasm
Transports amino acids to the mRNA
Have anticodons that are complementary to mRNA codons
Recognizes the appropriate codons on the mRNA and bonds to them with Hydrogen bonds
(there is a different version of tRNA molecule that matches each codon)
Ribosome
Coordinates the functioning of mRNA and tRNA
Consists of two subunits made up of proteins and rRNA (ribosomal RNA)
Small subunit with a binding site for mRNA
Large subunit with a binding site for tRNA
rRNA= made in nucleolus, and functions to decode the mRNA into amino acids
The subunits of the ribosome act as a vise holding the mRNA and tRNA close together
The ribosome connects the newly arrived amino acid to the growing polypeptide chain
Mutations
A. Mutation = permanent alteration in cell’s DNA base sequence
B. Almost all cancers begin as a mutation that is passed along at replication.
1.
In somatic cells (body cells)
2.
Mutation rate is low, but after decades of accumulated mutations, cells can become malignant.
C.
Heritable mutations occur in germ-line cells (cells that divide to make sperm and eggs).
D.
Heritable mutations also create genetic diversity.