Chapter 28: Chromosomes

The length of nucleic acid is much greater than the dimensions of the surrounding compartment. Chapter 28: Chromosomes Chromatin describes the state of nuclear DNA and its associated proteins during the interphase (between mitoses) of the cell cycle. A chromosome is a discrete unit of the genome carrying many genes. Each chromosome consists of a very long molecule of duplex DNA and an approximately equal mass of proteins. It is visible as a morphological entity only during mitosis. 1 28.4 Eukaryotic DNA Has Loops and Domains Attached to a Scaffold 2 28.5 Chromatin Is Divided Into Euchromatin and Heterochromatin • A chromosome scaffold is • Euchromatin composes all of the genome in the interphase nucleus except for the heterochromatin. – a proteinaceous structure – Not made up of histone proteins. • The euchromatin is less tightly coiled than heterochromatin, and contains the active or potentially active genes. • A matrix attachment region (MAR) is – a region of DNA that attaches to the proteinaceous scaffold. – It is also known as a scaffold attachment region (SAR). • MARs are A · T-rich but do not have any specific consensus sequence. – Usually contain consensus sequence for topoisomerase II – Many transcription factors also bind to MARs or adjacent to MARs 3 • Heterochromatin describes regions of the genome that are highly condensed, are not transcribed, and are late-replicating. • Heterochromatin is divided into two types, which are called constitutive and facultative. 4 1 Chromatin 28.5 Chromatin Is Divided Into Euchromatin and Heterochromatin Complex of DNA, histones, and non-histone proteins •Two broad classes of chromatin: –Euchromatin: –Majority chromatin is in its extended (decondensed) state during interphase –only condenses during mitosis. • Individual chromosomes can be seen only during mitosis. • • During interphase, the general mass of chromatin is in the form of euchromatin, – which is less tightly packed than mitotic chromosomes. –Heterochromatin: –Remains highly condensed even in interphase. –Accounts for the dark staining regions seen in interphase chromatin. • Regions of heterochromatin remain densely packed throughout interphase. (constitutive) •Constitutive: always inactive and condensed: –e.g. repetitive DNA, centromeric DNA •Facultative: can exist in both forms. –e.g.: Female X chromosome in mammals. 5 6 How chromosomal packaging influences gene activity • Decompaction precedes gene expression – Boundary elements delimit areas of decompaction – Nucleosomes in the decompacted area unwind to allow initiation of transcription • Transcription factors (nonhistone proteins) unwind nucleosomes and dislodge histones at 5’ end of genes • Unwound portion is open to interaction with RNA polymerase which can recognize promotor and initiate gene expression 7 8 2 Studies using DNase identify decompacted regions Normal chromatin structure slows transcription Condensed DNA exposed DNA exposed DNA 9 Remodeling of chromatin mediates the activation of transcription 11 10 Hypercondensation over chromatin domains causes transcriptional silencing 12 3 28.6 Genomic imprinting Chromosomes Have Banding Patterns • G-bands are – generated on eukaryotic chromosomes by staining techniques – appear as a series of lateral striations. – They are used for karyotyping • (identifying chromosomal regions by the banding pattern). • Certain staining techniques cause the chromosomes to – have the appearance of a series of striations called G-bands. • The bands are lower in G · C content than the interbands. • Results from Heterochromatinization that selectively silences genes inherited from one parent • Genes found in heterochromatin are not available to the transcription machinery • Are not transcribed • Genes are concentrated in the G · C-rich interbands. • Mammalian DNA is ~40% G-C 13 14 15 4