Summary
Key terms
DNA Replication: The process in which a cell's entire DNA is copied.
DNA Helicase: The enzyme that breaks the hydrogen bonds holding the two DNA strands together during DNA replication.
DNA Polymerase: The enzyme that builds a new DNA strand during DNA replication.
Leading Strand: The DNA strand that DNA polymerase constructs in the 5'3' direction.
Lagging Strand: The DNA strand at the opposite side of the replication fork from the leading strand.
Primase: An enzyme that builds a short RNA primer on the lagging strand during DNA replication.
Okazaki Fragments: Short fragments of DNA that comprise the lagging strand.
Central Dogma of Molecular Biology: Doctrine that genetic instructions in DNA are copied by RNA, which carries them to a ribosome where they are used to synthesize a protein.
Gene Expression: Activating a particular gene to make a protein.
Regulatory Protein: Proteins that regulate DNA transcription.
Activators: Regulatory proteins that promote transcription.
Repressors: Regulatory proteins that prevent transcription.
Regulatory Elements: Regions of DNA where regulatory proteins can bind.
Promoter: Region of a gene where a RNA polymerase binds to initiate transcription of the gene.
RNA Polymerase: An enzyme that helps produce RNA during transcription.
Operon: Region of DNA containing genes that code for proteins with a particular function (usually for prokaryotic cells).
Operator: Part of the operon where regulatory proteins bind.
TATA Box: Regulatory element in the promoter of most cells.
Homeobox Genes: Genes that regulate development.
Protein Synthesis: Process in which cells make proteins that includes transcription of DNA and translation of mRNA.
Genetic Code: Universal code of three-base codons that encodes the genetic instructions for the amino acid sequence of proteins.
Codon: Group of three nitrogen bases in nucleic acids that makes up a code “word” of the genetic code and stands for an amino acid, start, or stop.
Transcription: Process in which genetic instructions in DNA are copied to form a complementary strand of mRNA.
RNA Polymerase: An enzyme that helps produce RNA during transcription.
Promoter Site: Region of a gene where a RNA polymerase binds to initiate transcription of the gene.
Introns: Non-coding regions of mRNA that are removed by splicing.
Exons: Coding regions.
Translation: Process in which genetic instructions in mRNA are “read” to synthesize a protein.
Messenger RNA (mRNA): Type of RNA that copies genetic instructions from DNA in the nucleus and carries them to ribosomes in the cytoplasm.
Ribosomal RNA (rRNA): Type of RNA that helps to form ribosomes and assemble proteins.
Transfer RNA (tRNA): Type of RNA that brings amino acids to ribosomes where they are joined together to form proteins.
Mutation: A change in the bases of DNA/RNA
Mutagen: Something that causes mutation.
Germline Mutation: A mutation in a gamete (can be passed onto offspring).
Somatic Mutation: Mutation that occurs in a somatic (non-gamete) cell.
Chromosomal Alteration: Mutation that changes chromosome structure.
Point Mutation: One nucleotide is changed.
Frameshift Mutation: When a nucleotide is deleted or inserted.
Genetic Disorder: Disease caused by gene mutations.
Bacteriophage: Virus which attacks bacteria.
Plasmids: A plasmid is a small, circular, double-stranded DNA molecule, which is distinct from chromosomal DNA and can replicate independently.
Vectors: A cloning vector is a small piece of DNA into which a foreign DNA fragment can be inserted.
Conjugation: Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells.
Transformation: Bacterial transformation may be referred to as a stable genetic change brought about by the uptake of naked DNA.
Transduction: Transduction is the process by which DNA is transferred from one bacterium to another by a virus.
Key concepts
- DNA and RNA: The Hershey and Chase experiment with bacteria and phages, provided the first strong evidence that the genetic material is DNA..Watson and Crick proposed that DNA is a double–stranded molecule, where it contains ATGC as nitrogen bases, where the specific pairs, A with T and G with C (Chargaff's rule) . In the case of RNA molecule, A with U (Uracil) and G with C. Here, thymine (T) is replaced with Uracil (U).
- DNA replication : DNA replication is semiconservative. According to the base pairing rules, the pairing molecule unwinds, and each strand then serves as a template for the synthesis of a new strand. It occurs in S–phase, built in 5' to 3' direction. Helicase unzips the double strand, DNA polymerase comes in and adds on the nucleotides. Proofreading enzymes minimize errors of process.
- Gene expression and regulation: A gene is "expressed" when it is being transcribed into mRNA (and translated into protein), and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. Genetic code is the triplet of nucleotides that codes for a particular amino acids: start codon= AUG, stop codon= UGA, UAA, UAG. Transcription is the process in which mRNA is synthesized on a DNA template. RNA processing is a process where introns (non–coding) are spliced out, exons (coding) glued together: 3' poly–A tail, 5' G cap. Translation is the process by which the mRNA specified sequence of amino acids is lined up on a ribosome for protein synthesis.
- DNA packaging: Chromatin structure is based on successive levels of DNA packing. Eukaryotic chromatin is composed mostly of DNA and histone proteins that bind to each other and to the DNA to form nucleosomes, the most basic units of DNA packing. Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. Additional folding leads to highly condensed chromatin of the metaphase chromosome. In interphase cells, most chromatin is less compacted (euchromatin), but some remains highly condensed (heterochromatin). Genes in highly compacted chromatin are not transcribed. Histone acetylation seems to loosen chromatin structure and thereby enhance transcription. DNA methylation is associated with reduced transcription.
- DNA mutations: Mutation is a sudden change in chromosomal DNA, i.e., in a strict sense it covers only those changes which alter the chemical structure of the gene at the molecular level. The products of proto–oncogenes and tumor–suppressor genes control cell division. A DNA change that makes a proto–oncogene excessively active converts it to an oncogene, which may promote excessive cell division and cancer. A tumor–suppressor gene encodes a protein that inhibits abnormal cell division. A mutation in such a gene that reduces the activity of its protein product may also lead to excessive cell division and possibly to cancer. The spread of cancer from one place to another place in the body is known as metastasis. A hyperactive version of a protein in a stimulatory pathway, such as Ras functions as an oncogene protein. A defective version of a protein in an inhibitory pathway, such as p53, fails to function as a tumor suppressor.
- Genetics of viruses and bacteria: A virus is a small nucleic acid genome enclosed in protein capsid and sometimes a membranous envelope containing viral proteins that help viruses enter cells. The genome of viruses may be DNA or RNA or single stranded or double–stranded. Viruses use host machinery like enzymes, proteins to reproduce in the host cell. Viruses reproduces by lytic cycle and lysogenic cycle. Lytic cycle is the one in which the virus is actively reproducing and kills the host cells. Lysogenic cycle is the one in which the virus lies dormant within the DNA of the host cell. Bacterial cells are prokaryotic cells; consists of one double–stranded circular DNA molecule; reproduce by binary fission (e.g; plasmid– extra circle of DNA present in bacteria that replicate independently of main chromosome. There are three different gene transfer mechanisms in bacteria– transformation: uptake of foreign DNA from the surrounding environment, transduction– movement of genes from one cell to another by phages, which are incorporated by crossover. In generalized transduction, lytic cycle accidentally places host DNA into a phage, which is brought to another cell. In specialized transduction– virus leaving lysogenic cycle brings host DNA with it into phage. Conjugation is the transfer of DNA between two bacterial cells connected by sex pili. The control of gene expression is vital to the proper and efficient functioning of an organism. In bacteria, operons are a major method of gene expression control. Ex; Lac operon.
Molecular Genetics
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