Lecture 06, 08, 09

Cell divisions halt progression through cell cycle by inhibiting specific cyclin-CDK complexes.

Meiosis consists of two divisions resulting in four haploid daughter cells.

Homologous chromosomes are separated in meiosis I, each daughter cell receiving one copy of each chromosome.

Chromosomes have two sister chromatids which are separated in meiosis II.

Both meiosis and mitosis are present in last eukaryotic common ancestor.

Prokaryotes undergo binary fission or alternative manners of division, all preceded by DNA replication.

In eukaryotes, mitotic divisions create progeny from multicellular organisms, and enable growth and repair.

Mitotic cell division includes mitosis (equational division) and meiosis (reductional division).

Mitosis: chromosomes align on metaphase plate, then split and distributed between two daughter cells.

Meiosis I: homologous chromosomes paired, then separated and distributed between two daughter cells.

Meiosis II: chromatids separated and distributed like mitosis.

In humans and other animals, meiosis produces four gametes.

In plants, meiosis produces spores which germinate into haploid vegetative phase.

Interphase precedes mitosis, meiosis, and cytokinesis.

Interphase consists of G1, S, and G2 phases.

G1: cell growth, preparation for DNA replication, checkpoints for cell size and DNA damage.

S phase: chromosomes replicated.

G2: final stages of growth, checkpoints for cell size and DNA replication, spindles synthesized.

Mitosis or meiosis occurs in M phase.

Cyclin and cyclin-dependent kinases control checkpoints, progression through interphase.

Interphase progression results from increased cyclin and cyclin-dependent kinase attachment.

Three checkpoints before M phase: G1-S, G2, and metaphase.

Most important checkpoint is G1-S, ensuring cell is prepared for DNA replication.

DNA is a long polymer made of repeating nucleotide units, each consisting of a sugar, a phosphate group, and a nucleobase.

The sugar in DNA is 2-deoxyribose, and the backbone is formed by phosphate groups connecting the third and fifth carbon atoms of adjacent sugars.

DNA usually exists as a double helix, with two antiparallel strands held together by hydrogen bonds and base-stacking interactions.

Humans have more than 98% non-coding DNA, and the remaining coding DNA specifies the order of amino acids in proteins.

RNA is produced using DNA as a template in a process called transcription, and it specifies the order of amino acids in proteins through the genetic code during translation.

Eukaryotic DNA is organized into chromosomes, while prokaryotic DNA is found in the cytoplasm in circular chromosomes.

Chromatin proteins help compact and organize DNA, guiding interactions between DNA and other proteins.

DNA strands have asymmetric ends, with a 5′ phosphate group and a 3′ hydroxyl group, and they have a buoyant density of 1.7g/cm3.

The nucleobases in DNA are classified into two groups: the purines (adenine and guanine) and the pyrimidines (cytosine, thymine, and uracil).

Modified bases, such as 5-methylcytosine, occur in some DNA molecules, and their presence can help bacteria avoid restriction enzymes.

DNA replication involves the unwinding of DNA at the origin and the synthesis of new strands through the action of an enzyme called helicase.

Replication forks grow bi-directionally from the origin, aided by various proteins.

DNA polymerase synthesizes new strands by adding nucleotides that complement the template strands.

DNA replication occurs during the S-stage of interphase.

Artificial DNA replication can be performed in vitro using DNA polymerases and primers.

Techniques like Polymerase chain reaction (PCR), ligase chain reaction (LCR), and transcription-mediased amplification (TMA) are used for in vitro DNA replication.

DNA is a double-stranded molecule made up of nucleotides.

Nucleotides consist of a sugar, a phosphate, and a nucleobase.

Adenine pairs with thymine, and guanine pairs with cytosine through hydrogen bonds.

DNA strands have a directionality, with the 5' and 3' ends referring to the carbon atom in deoxyribose to which the next phosphate in the chain attaches.

DNA polymerases extend existing strands by adding new nucleotides, aided by a primer.

DNA polymerization is driven by the hydrolysis of high-energy phosphate bonds.

DNA polymerases are highly accurate, with an error rate of less than one mistake for every 107 nucleotides added.

Proofreading and post-replication mismatch repair mechanisms ensure replication fidelity.

DNA replication proceeds in three steps: initiation, elongation, and termination.

The rate of DNA replication in a living cell is approximately 749 nucleotides per second.

The mutation rate per base pair per replication is 1.7 per 108.

The RNA world hypothesis suggests that early RNA molecules were the basis for catalyzing organic chemical reactions and self-replication.

Compartmentalization was necessary for chemical reactions to occur effectively and to differentiate reactions with the external environment.

Ribocells or ribocytes are hypothetical cells with RNA genomes instead of DNA, which may have been the earliest form of life.

Prokaryotes lack membrane-bound organelles and have a single, circular chromosome.

Prokaryotes have the ability to utilize a wide range of organic and inorganic materials for metabolism.

Bacteria are one of the oldest forms of life and can reproduce asexually through binary fission or sexually through natural genetic transformation.

Plasmids are circular, self-replicating DNA molecules that carry genes for novel abilities, including antibiotic resistance.

Photosynthetic cyanobacteria are the most successful bacteria and changed the early Earth's atmosphere by oxygenating it, leaving behind extensive fossil records in the form of stromatolites.

Archaea are phylogenetically related to eukaryotes and have significant molecular differences, most notably in their membrane structure and ribosomal RNA.

Archaea inhabit extreme environments and can be classified as thermophiles, psychrophiles, alkaliphiles, acidophiles, and piezophiles.

Methanogens are a subset of archaea that can produce methane and are significant in wetland environments as well as in the digestive systems of animals.

Eukaryotic cells contain membrane-bound organelles, such as mitochondria, a nucleus, and chloroplasts.

Prokaryotic cells may have transitioned into eukaryotic cells between 2 and 2.5 billion years ago.