How Cells Divide 10 - Bacterial Cell Division Bacteria...

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Bacterial Cell Division - Bacteria divide by binary fission - No sexual life cycle - Reproduction is clonal - Single, circular bacterial chromosome is replicated - Replication begins at the origin of replication and proceeds in two directions to site of termination - New chromosomes are partitioned to opposite ends of the cell - Septum forms to divide the cells into 2 cells - Septation production of septum separates cell’s other components - Begins with formation of ring of FtsZ proteins - Accumulation of other proteins follow - Structure contracts radially to pinch cell in two - FtsZ protein found in most prokaryotes Eukaryotic Chromosomes - Every species has a different number of chromosomes - Humans have 46 chromosomes in 23 nearly identical pairs - Additional/missing chromosomes usually fatal Chromosomes composed of chromatin – complex of DNA and protein - DNA of single chromosome is one long continuous double-stranded fiber - RNA associated with chromosomes during RNA synthesis - Typical human chromosome 140 million nucleotides long - In the nondividing nucleus - Heterochromatin not expressed - Euchromatin expressed Chromosome Structure - Nucleosome complex of DNA and histone proteins - Promote and guide coiling of DNA - DNA duplex coiled around 8 histone proteins every 200 nucleotides - Histones are positively charged and strongly attracted to negatively charged phosphate groups of DNA - Solenoids nucleosomes wrapped into higher order coils - Leads to a fiber 30nm in diameter - Usual state of nondividing (interphase) chromatin - During mitosis, chromatin in solenoid arranged around scaffold of protein to achieve maximum compaction - Radial looping aided by condensin proteins
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Karyotype particular array of chromosomes in an individual organism - Arranged according to size, staining properties, location of centromere, etc. - Humans are diploid ( 2n ) - 2 complete sets of chromosomes - 46 total chromosomes - Haploid ( n ) – 1 set of chromosomes - 23 in humans - Pair of chromosomes are homologous - Each one is a homologue Replication - Prior to replication, each chromosome composed of a single DNA molecule - After replication, each chromosome composed of 2 identical DNA molecules - Held together by cohesin proteins - Visible as 2 strands held together as chromosome becomes more condensed
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