The importance of telomerase in maintaining chromosome integrity

The Nucleus of a Eukaryotic Cell (18) Centromeres – The centromere is located at the site markedly indented on a chromosome. – Centromeres contain constitutive heterochromatin. – Centromeric DNA is the site of microtubule attachment during mitosis. – DNA sequence is not important for centromere structure and function.

A centromere is marked by a distinct indentation

The Nucleus of a Eukaryotic Cell (19) Epigenetics: There’s More to Inheritance than DNA – Epigenetic inheritance depends on factors other than DNA sequences. – Parental histones determine the chemical modifications found in the newly synthesized histones.

The Nucleus of a Eukaryotic Cell (20) The Nucleus as an Organized Organelle – Chromatin fibers are concentrated at specific domains within the nucleus.

The Nucleus of a Eukaryotic Cell (21) Chromosome ordering is directed by the nuclear envelope proteins. In the nucleus, mRNAs are synthesized as discrete sites. DNA sequences that participate in a common biological response but reside on different chromosomes interact within the nucleus.

Interaction between distantly located genes

Interaction between distant genes (continued)

Nuclear compartmentalization of the cell’s mRNA processing machinery

The Nucleus of a Eukaryotic Cell (22) The Nuclear Matrix – The nuclear matrix is a network of protein- containing fibrils. – It serves as more than a skeleton to maintain the shape of the nucleus and anchoring the machinery involved in nuclear activities.

The nuclear matrix

The Human Perspective: Chromosomal Aberrations and Human Disorders (1) A chromosomal aberration is loss or exchange of a segment between different chromosomes, caused by exposure to DNA- damaging agents. Chromosomal aberrations have different consequences depending on whether they are in somatic or germ cells.

The Human Perspective: Chromosomal Aberrations and Human Disorders (2) Inversions involve the breakage of a chromosome and resealing of the segment in a reverse order.

The Human Perspective: Chromosomal Aberrations and Human Disorders (3) Translocations are the result of the attachment of all or one piece of one chromosome to another chromosome.

The Human Perspective: Chromosomal Aberrations and Human Disorders (4) Deletions result when thee is loss of a portion of a chromosome. Duplications occur when a portion of a chromosome is repeated.

12.2 Control of Gene Expression in Bacteria (1) Bacterial cells selectively express genes to use the available resources effectively. – The presence of lactose in the medium indices the synthesis of the enzyme β-galactosidase. – The presence of tryptophan in the medium represses the genes that encode enzymes for tryptophan synthesis.

The kinetics of β-galactosidase induction

Control of Gene Expression in Bacteria (2) The Bacterial Operon – An operon is a functional complex of genes containing the information for enzymes of a metabolic pathway. It includes: Structural genes – code for the enzymes and are translated from a single mRNA. Promoter – where the RNA polymerase binds. Operator – site next to the promoter, where the regulatory protein can bind.

Control of Gene Expression in Bacteria (3) The Bacterial Operon (continued) – An operon includes: A repressor which binds to a specific DNA sequence to determine whether or not a particular gene is transcribed. The regulatory gene encodes the repressor protein.

Organization of a bacterial operon

Gene regulation by operons

Control of Gene Expression in Bacteria (4) The lac Operon – It is an inducible operon, which is turned on in the presence of lactose (inducer). The lac operon contains three structural genes. Lactose binds to the repressor, changing its conformation and making it unable to bind to the operator. A repressor protein can bind to the operator and prevent transcription in the absence of lactose.

Control of Gene Expression in Bacteria (5) The lac Operon (continued) – Positive Control by Cyclic AMP The lac repressor exerts negative control. The glucose effect is an example of positive control. Cyclic AMP (cAMP) acts by binding to a cAMP receptor protein (CRP). Binding of CRP-cAMP to the lac control region changes the conformation of DNA thus allowing RNA polymerase to transcribe the lac operon.

The nucleotide sequence of binding sites in the control region of the lac operon

Control of Gene Expression in Bacteria (6) The trp Operon – It is a repressible operon, which is turned off in the presence of tryptophan. – The trp operon repressor is active only when it is bound to a corepressor such as tryptophan.

Control of Gene Expression in Bacteria (7) Riboswitches – A number of bacterial mRNAS can bind to a small metabolite, which in turn alters the gene involved in the production of such metabolite. – These mRNAs are called riboswitches because they undergo a conformational change and can suppress gene expression. – Riboswitches allow bacteria to regulate gene expression in response to some metabolites.

12.3 Control of Gene Expression in Eukaryotes (1) Cells of a complex eukaryote exist in many differentiated states. – Differentiated cells retain a full set of genes. – Nuclei from cells of adult animals are capable of supporting the development of anew individual, as demonstrated in experiments.

Cloning demonstrates that nuclei retain a complete set of genetic information

Control of Gene Expression in Eukaryotes (2) Genes are turned on and off as a result of interaction with regulatory proteins. – Each cell type contains a unique set of proteins. – Regulation of gene expression occurs on three levels: Transcriptional-level control Processing-level control Translational-level control

Overview of levels of control of gene expression

12.4 Transcriptional-level control (1) 1. Differential transcription is the most important mechanism by which eukaryotic cells determine which proteins are synthesized.

Transcriptional-level control (2) 2. DNA microarrays can monitor the expression of thousands of genes simultaneously. a) Immobilized fragments of DNA are hybridized with fluorescent cDNAs. b) Genes that are expressed show up as fluorescent spots on immobilized genes. c) Microarrays a provide a visual picture of gene expression.

The construction of a DNA microarray

DNA microarrays and their use in monitoring gene transcription

Transcriptional-level control (3) The Role of Transcription Factors in Regulating Gene Expression – Transcription factors are the proteins that either acts as transcription activators or transcription inhibitors. A single gene can be controlled by different regulatory proteins. A single DNA-binding protein may control the expression of many different genes.

Interactions between transcription factors bound to different regions of a gene

Transcriptional-level control (4) The Role of Transcription Factors in Determining a Cell’s Phenotype – Embryonic stem (ES) cells and are pluripotent, capable of differentiating into all of the different types of cells. – The importance of transcription factors in ES cells was demonstrated when these factors were introduced and shown to reprogram these cells.

Transcriptional-level control (5) The Structure of Transcription Factors – Transcription factors contain a DNA-binding domain and an activation domain. Transcription Factor Motifs – The DNA-binding domains of most transcription factors have related structures (motifs) that interact with DNA sequences. – Most of the motifs contain a segment that binds to the major groove of the DNA.

Interaction between a transcription factor and its DNA target sequence

Transcriptional-level control (6) Transcription factors (continued) – The zinc finger motif – the zinc ion of each finger is held in place by two cysteines and two histidines.

Transcriptional-level control (7) Transcription Factors (continued) – The helix-loop-helix (HLH) motif – has two α-helical segments separated by a loop. – The leucine zipper motif – has a leucine at every seventh amino acid of an α-helix.

Transcriptional-level control (8) HLH-containing transcription factors play a key role in the differentiation of certain tissues. HLH-containing transcription factors also participate in the control of cell proliferation and cancer.

Transcriptional-level control (9) DNA Sites Involved in Regulating Transcription – The TATA box regulates the initiation of transcription. – The core promoter, from the TATA box to the start, is where the initiation complex assembles. – The CAAT and the GC box are upstream and are required for initiation. – Alternative promoters allow some genes to be transcribed at more than one site.

Identifying promoter sequences