Intro to Molecular Genetics
Intro to Molecular Genetics
DNA has instructions for proteins your body makes and proteins determine function and structure of all cells. Amino acids make up proteins.
DNA is in chromosomes and chromosomes are always inside the nucleus in eukaryotes.
RNA is a molecule that can get through pores in the nuclear membrane. It takes information from DNA into nucleus then to a ribosome in the cytoplasm and help make proteins.
The sequence of events being discovered: DNA, RNA, and Protein --> is central dogma of molecular biology and the processes of transcription and translation are involved.
DNA is made up of nucleotides which have a nitrogen-containing base (five-carbon sugar [deoxyribose]) and a phosphate group. These nucleotides can only have one of these four bases: adenine (A), guanine (G), cytosine (C), or thymine (T). Cytosine and thymine are pyrimidine bases and guanine and adenine are purine bases.
Erwin Chargaff (1905-2002) analyzed base composition of DNA & proposed two main rules.
In DNA, the amount of one base (purine), always almost equals amount of a certain second base (a pyrimidine).
Human DNA is 30.9% A, 29.4% T, 19.9% G, and 19.8% C. This forms the basis of base pairs in double helix of DNA--> A&T and C&G. Number of pyrimidines (C&G) almost equals amount of purines (A&T), which is a consequence of base-pairing nature of DNA double-helix.
Chargaff showed that composition of DNA varied by species (in terms of relative amount of A, C, G, and T bases). The molecular diversity supported idea that DNA could be genetic material.
DNA Structure and Replication
DNA replication: when DNA is copied which occurs during the (S) phase of a eukaryotic cell cycle. Since the DNA strand is a complete copy, both strands of the double-helix can be templates for making of new strand. DNA replication is semi-conservative: resulting double-helix has an old and new strand, both.
Three models proposed for replication:
Semi-conservative replication: two copies; one new strand and one old one [most reasonable because it allowed each daughter strand to remain associated w/ template strand.
conservative replication: two original DNA strands would remain together in double-helix and two new ones would be for new DNA.
dispersive replication: two copies of DNA w/ a mix of old & new material
The Meselson-Stahl Experiment
Matthew Meselson and Franklin Stahl had evidence for semiconservative DNA in 1958. Experiment used fact that there are two forms of nitrogen available: 14N and a heavier isotope, 15N. DNA of cells in 15N medium had higher density
E. coli was grown for several generations with 15N. 2. E coli cells with 15N in DNA went to 14N medium and allowed to divide. 3. After one replication, DNA had intermediate density between 15N and 14N. Since conservative replication would have equal amounts of DNA in 15N and 14N densities, conservative replication model was excluded. 4. Another round of replication in 14N medium since original 15N DNA would have been split evenly among all DNA strands.
Helicase and Polymerase
DNA replication starts as enzyme: DNA helicase that breaks hydrogen bonds holding the two strands together and makes a replication fork. Structure then has two branching strands of DNA backbone with exposed bases. Exposed bases are read by enzyme, DNA polymerase which builds complementary DNA strand and DNA helicase continues to open double helix and grow replication fork.
Two new strands of DNA are built in opposite directions, leading or lagging strand.
Leading strand: DNA polymerase constructs in 5' --> 3' direction (continuous matter)
Lagging strand: DNA polymerase cannot construct in 5' --> 3' direction and makes short, Okazaki fragments. [primase makes short RNA primer > DNA polymerase can now make DNA in 5' --> 3' direction b/c can use free 3 OH group > DNA ligase attaches nucleotides together
replication forks mostly develop along a chromosome and continue until forks meet
DNA>RNA>Protein (central dogma of molecular biology)
"DNA makes RNA make protein" = DNA is transferred to RNA and used by RNA to make proteins
gene: segment of DNA with information to encode RNA molecule or protein and are embedded within DNA in nucleus (DNA never leaves nucleus) but RNA is relatively small and can carry info. outside of nucleus (occurs when gene product is needed by cell)
Ribosomes are site of protein synthesis
Even with thousands of genes, not all are used in every cell type
Gene expression: only occurs when gene product is needed by cell. Otherwise, if all genes were expressed all the time, cells would be similar and would be a waste of energy.
Transcription: process that transcribes information in a gene into an RNA molecule (beginning of gene expression process)