Replication transcription translation

Question 1

DNA Replication

Answer 1

Each time a cell divides, the entire DNA content of that cell must be duplicated so that the total complement of hereditary information (genome) is retained in each daughter cell

semi-conservative fashion with the two parent strands
of a dsDNA molecule each serving as the template for the synthesis of a daughter
strand

For replication to occur, the original double stranded helix must be
separated
• This is an energetically unfavorable event ( break hydrogen bonds)
• Accomplished by a combination of DNA specific proteins and enzymes

Question 2

DNA polymerase lll

Answer 2

enzyme that synthesizes daughter strands of DNA
• Reads the parent template and attaches nucleotides to the growing daughter
strand according to the base-pairing rules of dsDNA

Begins synthesis at the replication fork (the point of strand separation) with a short RNA primer that base-pairs to the parent template
• This primer will eventually be excised and replaced with DNA by the DNA repair enzyme DNA polymerase I

Only synthesizes DNA in the 5’-3’ direction

continuous strand is called the leading strand

The Okazaki fragments of the discontinuous strand are then
joined together by the DNA ligase enzyme
• The discontinuous strand is called the lagging strand

Question 3

rolls of other proteins in DNA replication

Answer 3

Many other proteins are involved in:
• Unwinding and stabilizing the parent strandsfor synthesis
• Protecting single-stranded regions
• Recognizing initiation sites
• Synthesizing the RNA primer

Question 4

ensuring accuracy while DNA replicating

Answer 4

DNA polymerases possess an exonuclease or
“proof-reading” function which is important for
maintaining the integrity of the DNA sequence

When an incorrect nucleotide is added a conformational
change brings the chain in contact with the
exonuclease portion of the enzyme which removes the incorrect nucleotide

Question 5

Transcription

Answer 5

Regulation of transcription is the primary mechanism cells use to control gene expression

process of transferring sequence information from the gene regions of DNA into a mRNA molecule
• Copying one strand of DNA into RNA
• Catalyzed by RNA polymerase
• Occurs mostly during interphase

Transcription continues until chain termination occurs in
response to specific sequences

RNA transcript quickly detaches from the template
DNA because the restoration of the DNA-DNA duplex is
energetically more factorable

end-product is a complementary sequence of
ribonucleotides that contain the information necessary for
protein synthesis ( or other function)

Question 6

RNA polymerase ll

Answer 6

in transcription; Binds to sequences in the regulatory region of the gene called the promote

Initiation of transcription requires many protein cofactors to bind to RNA polymerase to form the active
initiation complex

Question 7

Promoters

Answer 7

approximately 100 bases away from the initiation site of transcription where the first ribonucleotide
unit is paired with the template
• Rich in thymine and adenine in repeating patterns
• Sometimes referred to as a TATA box

Question 8

Enhancers

Answer 8

Other regions of DNA which may interact with the initiation complex to stimulate or repress transcription

Question 9

RNA processing- end stage of transcription

Answer 9

Additional modifications are required before mRNA can be exported to the cytoplasm
• Then 5’ end is modified by the addition of 7-methyl guanosine residues to form a structure called a cap

• The 3’ end is modified by polyadenylation the addition of multiple adenine bases, called the poly A tail
• These caps and tails are necessary for translation of mRNA into a protein and may help stabilize the mRNA molecule

Introns are removed from the mRNA by a molecular complex termed a spliceosome
• These complexes are composed of multiple small nuclear ribonucleoprotein particles (snRNPs)
• Spliceosomes mediate the cleavage and ligation of RNA at specific recognition sequences
• After the introns are removed, the mRNA contains exons and can be transported into the cytoplasm

Question 10

introns vs exons

Answer 10

Intron
• A noncoding region of a gene, located between exons
• Will NOT be translated into protein
• Spliced out by spliceosomes during mRNA processing
• Not found in prokaryotes

Exon
• The coding region of a gene that will be expressed as protein following translation
• Found in prokaryotes and eukaryotes

Question 11

when does splicing occur

Answer 11

during the end of transcription in eukaryotic cells only bc prokaryotic cells don’t have introns

Question 12

mRNA Translation

Answer 12

mature mRNA is transported from the nucleus to the cytoplasm

• Translation is the process whereby an mRNA sequence directs the amino acid sequence during protein synthesis

Translation takes place on ribosomes

• A ribosome binds to the initiation site on the mRNA to for an initiation complex
• During synthesis, codons are “read” by transfer RNA (tRNA) tRNA anticodon base-pairs with the next mRNA codon

An enzyme on the ribosome then catalyzes the formation of a peptide bond between the amino acid bound to the tRNA and the growing amino acid chain

• The previous tRNA is released and the next tRNA is added
• The ribosome moves along the mRNA until a stop codon is reached
• The ribosome and protein product are then dissociated from the mRNA
• More than one ribosome can move along an mRNA molecule at a time, forming a polyribosome

Question 13

Codon

Answer 13

three-nucleotide sequence that “codes” for an amino acid during translation (protein synthesis)
• There are 64 possible codons
• While most codons code for a specific amino acid, two codons signal termination of protein synthesis – called stop codons

• One additional codon (UGA) can code for either a stop or a selenocysteine, depending on the adjacent sequences and proteins.

• There are twenty-one amino acids involved in protein synthesis
• Each protein is specified by one or more codons