college bio DNA replication, translation transcription

The DNA replication process is semiconservative, which results in two DNA molecules,

each having one parental strand of DNA and one newly synthesized strand.

• In bacteria, the initiation of replication occurs at the origin of replication,

where supercoiled DNA is unwound by DNA gyrase, made single-stranded by helicase,

and bound by single-stranded binding protein to maintain its single-stranded

state. Primase synthesizes a short RNA primer, providing a free 3’-OH group to

which DNA polymerase III can add DNA nucleotides.

• During elongation, the leading strand of DNA is synthesized continuously from a single

primer. The lagging strand is synthesized discontinuously in short Okazaki fragments,

each requiring its own primer. The RNA primers are removed and replaced with DNA

nucleotides by bacterial DNA polymerase I, and DNA ligase seals the gaps between

these fragments.

• During transcription, the information encoded in DNA is used to make RNA.

• RNA polymerase synthesizes RNA, using the antisense strand of the DNA as template by

adding complementary RNA nucleotides to the 3’ end of the growing strand.

• RNA polymerase binds to DNA at a sequence called a promoter during the initiation of

transcription.

• Unlike DNA polymerase, RNA polymerase does not require a 3’-OH group to add

nucleotides, so a primer is not needed during initiation.

• Termination of transcription in bacteria occurs when the RNA polymerase encounters

specific DNA sequences that lead to stalling of the polymerase. This results in release of

RNA polymerase from the DNA template strand, freeing the RNA transcript.

• Eukaryotic pre-mRNAs are modified with a 5' methylguanosine cap and a poly-A tail.

These structures protect the mature mRNA from degradation and help export it from

the nucleus.

• Pre-mRNAs also undergo splicing, in which introns are removed and exons are

reconnected with single-nucleotide accuracy. Only finished mRNAs that have

undergone 5' capping, 3' polyadenylation, and intron splicing are exported from the

nucleus to the cytoplasm.

• In translation, polypeptides are synthesized using mRNA sequences and cellular

machinery, including tRNAs that match mRNA codons to specific amino acids and

ribosomes composed of RNA and proteins that catalyze the reaction.

• The genetic code is degenerate in that several mRNA codons code for the same amino

acids. The genetic code is almost universal among living organisms.

• Prokaryotic (70S) and cytoplasmic eukaryotic (80S) ribosomes are each composed of a

large subunit and a small subunit of differing sizes between the two groups. Each

subunit is composed of rRNA and protein. Organelle ribosomes in eukaryotic cells

resemble prokaryotic ribosomes.

• Some 60 to 90 species of tRNA exist in bacteria. Each tRNA has a three-

nucleotide anticodon as well as a binding site for an amino acid. All tRNAs with a

specific anticodon will carry the same amino acid.

• Initiation of translation occurs when the small ribosomal subunit binds with initiation

factors and an initiator tRNA at the start codon of an mRNA, followed by the binding to

the initiation complex of the large ribosomal subunit.

• In prokaryotic cells, the start codon codes for N-formyl-methionine carried by a special

initiator tRNA. In eukaryotic cells, the start codon codes for methionine carried by a

special initiator tRNA.

• During the elongation stage of translation, a charged tRNA binds to mRNA in the A

site of the ribosome; a peptide bond is catalyzed between the two adjacent amino

acids, breaking the bond between the first amino acid and itstRNA;the ribosome moves

one codon along the mRNA; and the first tRNA is moved from the P site of the ribosome

to the E site and leaves the ribosomal complex.

• Termination of translation occurs when the ribosome encounters a stop codon,

which does not code for a tRNA. Release factors cause the polypeptide to be

released, and the ribosomal complex dissociates.

• In prokaryotes, transcription and translation may be coupled, with translation of an

mRNA molecule beginning as soon as transcription allows enough mRNA exposure

for the binding of a ribosome, prior to transcription termination.

• A mutation is a heritable change in DNA. A mutation may lead to a change in the

amino- acid sequence of a protein, possibly affecting its function.

• A point mutation affects a single base pair. A point mutation may cause a silent

mutation if the mRNA codon codes for the same amino acid, a missense mutation if

the mRNA codon codes for a different amino acid, or a nonsense mutation if the

mRNA codon becomes a stop codon.

• Missense mutations may retain function, depending on the chemistry of the new

amino acid and its location in the protein. Nonsense mutations produce truncated

and frequently nonfunctional proteins.

• A frameshift mutation results from an insertion or deletion of a number of

nucleotides that is not a multiple of three. The change in reading frame alters every

amino acid after the point of the mutation and results in a nonfunctional protein.

• Spontaneous mutations occur through DNA replication errors, whereas

induced mutations occur through exposure to a mutagen.

• Mutagenic agents are frequently carcinogenic but not always. However, nearly

all carcinogens are mutagenic.

• Ionizing radiation, such as X-rays and γ-rays, leads to breakage of the

phosphodiester backbone of DNA and can also chemically modify bases to alter their

base-pairing rules.

• Nonionizing radiation like ultraviolet light may introduce pyrimidine (thymine)

dimers, which, during DNA replication and transcription, may introduce frameshift

or point mutations.