2.7 DNA Replication, Transcription and Translation Flashcards
What does semi-conservative mean?
DNA replication is semi conservative as each of the new chromatids contains a strand of the original DNA and one new strand
Why is DNA replicated?
to produce an exact copt of a chromosome in preparation for cell division
a new daughter cell must have a complete copy of the genetic code
Steps of DNA replication
the DNA is uncoiled
then unwound by DNA gyrase (straightens out)
helicase then breaks the weak H bonds between bases, unzipping it
new pieces of DNA are formed from free nucleotides joined by DNA polymerase
the free nucleotides are matched up to complementary nucleotides
How did Meselson and Stahl prove semi-conservative nature of DNA?
they grew cells for many generations in a medium containing only heavy nitrogen (N15)
when centrifuged it created a low band
then they transferred the cells to a medium containing only light nitrogen (N14)
after one generation the hybrid band was at a higher position when centrifuged
after a second generation two bands were formed one for the hybrid and one for the light nitrogen
showing semi conservative nature of DNA
How are base mutations reduced?
more than one triplet codes for one amino acid (in some cases)
many of the codons for a single amino acid differ only in the last base reducing the chance that base mutations will have any noticeable affect
Definition of Transcription
it is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase
Steps of Transcription
a section of DNA unwinds inside the nucleus
one side of the DNA acts as a template on which the complementary molecule of mRNA is made
process is catalysed by RNA Polymerase and only occurs in the 5’ to 3’ direction
the bases that would be thymine are replaced by uracil
the mRNA then moves to the ribosomes
(only the exons are copied - the introns are edited out)
Definition of Translation
it is the synthesis of polypeptides on ribosomes
the amino acid sequence of polypeptides is determined by mRNA according to the genetic code
codons of three bases on mRNA correspond to one amino acid in a polypeptide
translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA
translation is the process of building a polypeptide chain from amino acids, guided by the sequence of codons on the mRNA
What are the structures involved in translation?
- mRNA: carries the code from the DNA that will be translated into an amino acid sequence
- tRNA: transports amino acids to their correct position on the mRNA strand. Each amino acid has a specific tRNA activating enzyme
- Amino Acids: from which the polypeptide is constructed
- Ribosomes: provide the environment for tRNA attachment and amino acid linkage
Structure of tRNA molecule
four sections:
1. carrier end: for binding to the specific amino acid (up top)
2. anticodon site: for binding to the codon of the mRNA
3. ribosome site: for attaching to the ribosome
4. enzyme site: for the tRNA activating enzyme
