Genetics Learning Objectives Part 1: Molecular and Genomic Levels
Last updated: 10/28/11
DNA as the genetic material
• Essential properties of deoxyribonucleic acid (DNA) as genetic material: stable, can be
copied accurately, contains information, can be transmitted.
• DNA is a component of chromosomes
• Chromosomes, composed of protein and DNA, are physical units of heredity (demonstrated
in Hershey-Chase experiment; DNA but not protein enters bacteria when bacteriophage
• DNA is a polymer
of monomeric units called
nucleotides. A nucleotide is composed of
a 5 carbon sugar, deoxyribose.
triphosphate group, attached to the sugar at the 5 prime carbon.
3. a nitrogen-containing base, attached to the sugar at
the 1 prime carbon.
• There are 4 nitrogenous bases in DNA: two 1-ringed (C and T), two 2-ringed (G and A)
• single stranded DNA = a chain of nucleotides, covalently bonded,
grows in one direction
only (5’ phosphate group attaches to 3’ carbon).
• Chargaff’s rule:
in all species examined, proportions of A=T and proportions of
• Watson and Crick (Franklin): DNA is double-stranded, hydrogen bonds hold the two strands
• Two DNA strands are antiparallel and form a helix
Basic mechanism of DNA replication.
• DNA is copied – replicated – using a semi-conservative model (demonstrated in MeselsonStahl
• Bacteria have one circular double-stranded DNA chromosome, with one origin of replication.
• In eukaryotes, large linear chromosomes contain many origins of replication used at the same
• Steps in replication: helix unwinds - a replication bubble forms.
Replication fork = junction
between the open and closed regions, replication bubble = open DNA between two adjacent
• Helicase unwinds DNA: single strand binding proteins (SSBPs) keep it single stranded:.
makes a short RNA copy of DNA
• DNA polymerase extends the chain, and then replaces RNA primers with DNA.
• DNA polymerase cannot initiate a new strand, it can only add onto an existing 3’ OH group.
• Only the leading strand is synthesized continuously toward the replication fork.
opposite strand (lagging strand) is made away from the replication fork, in small fragments
• DNA ligase makes covalent bonds between adjacent Okazaki fragments
• Simple definition of a gene: one gene makes one protein =
the central dogma of biology.
• Gene --> protein --> phenotype
• DNA produces protein indirectly,
an intermediate molecule – ribonucleic acid,
• In bacteria, transcription and translation are coupled (i.e. can occur almost simultaneously)
• Eukaryotic cells
uncouple transcription and translation: transcription takes place in the
nucleus, translation in the cytoplasm.
RNA and the basic mechanism of transcription