bio211 chapter 18 notes

What is the advantage of grouping similar functional genes together?

A single “on-off” switch can control the whole cluster of genes

What is an operator?

On-off switch for a segment of DNA

• positioned within the promoter or between the promoter and enzyme-coding genes

• Controls access of RNA polymerase to genes

What is an operon?

Operator, promoter, and the genes that they control

What is a repressor?

Binds to the operator, preventing RNA polymerase from transcribing genes and from binding

What is a regulatory gene?

A gene that is encoded for a repressor protein

How do repressor proteins work?

Repressors are made in an inactive state, and are allosteric→ It can only bind to the operator when a specific molecule binds to it

What is a corepressor?

A small molecule that cooperates with a repressor protein to switch an operon off

What is an example of corepressors?

1. As tryptophan accumulates, more tryptophan molecules associate with trip repressor molecules, which can then bind to the trp operator and shut down production of the tryptophan pathway enzymes

2. If the cells tryptophan levels drop, there would be less repressor proteins with tryptophan, making them inactive

What is an inducible operon?

An operon that is inactive but can be stimulated to an active state when a specific small molecule interacts with a different regulatory protein

What is an activator?

A protein that binds to DNA and stimulates transcription of a gene

What is differential gene expression?

The expression of different genes by cells w/ the same genome

What is histone acetylation?

Addition of an acetyl group to an amino acid in a histone tail→ promotes transcription by opening up chromatin structure

What is DNA methylation?

Enzymes can methylate the DNA itself rather than modifying histone proteins

What is epigenetics?

inheritance of traits transmitted by mechanisms not involving the nucleotide sequence

What are control elements?

Segments of noncoding DNA that serve as a binding sites for the proteins called transcription factors→ bind to the control elements and regulate transcription

What are enhancers?

Groupings of distant control elements that are thousands of nucleotides up/downstream

What is the process of transcription activators and enhancers?

1. Activator proteins bind to distal enhancers in the DNA→ all the control segments make up one enhancer

2. A DNA-bending protein brings the bound activators closer to the promoter

3. The activators bind to certain mediator proteins and general transcription factors, helping them form an active transcription initiation complex on the promoter

What serves as a transcription activator in a eukaryotic cell?

A steroid hormone→ enters cell and binds to a specific intracellular receptor protein, forming a hormone-receptor complex

• every gene that is transcribed as a result from a given steroid hormone has a control elements recognized by the hormone-receptor complex

What is alternative RNA splicing?

Different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons/introns

What are miRNAs?

MicroRNAs→ small, single-stranded RNA molecules capable of binding of binding to complementary sequences in mRNA molecules

• A longer RNA precursor is processed by cellular enzymes into an miRNA, a single-stranded RNA of about 22 nucleotides that forms a complex w/ one or more proteins

• Allows complex to bind to any mRNA molecule

What regulates gene expression by miRNAs?

If miRNA and mRNA bases are complementary all along their length, the mRNA is degraded; if the match is less than complete, the translation is blocked

What are siRNAs?

Small interfering RNAs

• differs from miRNAs based on structure of precursors

What is RNA interference?

RNAi- blocking of gene expression by siRNAs

What are lncRNAs?

Long noncoding RNAs

• multitude of purposes

What is morphogenesis?

The development of the form of an organism and its structures

What are cytoplasmic determinants?

Maternal substances in the egg that influence the course of early development of the future embryo in many species

What is induction?

signals conveyed to an embryonic cell from other embryonic cells in the vicinity→ causes change in gene expression that lead to observable cellular changes

What is determination?

refers to the point at which an embryonic cell is irreversibly committed to becoming a particular cell type

• once it has undergone determination, an embryonic cell can be experimentally placed in another location in the embryo and thus still differentiate

How do myoblasts differentiate?

The myoD gene is the master regulatory gene→ capable of differentiating cells into muscle cells

What is pattern formation?

Cytoplasmic determinants and inductive signals contribute to spatially organizing tissue/organs of an organism in their characteristic places

What is positional information?

Cytoplasmic determinants and inductive signals cue a cell its location relative to body axes and to neighboring cell,s determining how the cell and its descendants will respond to future molecular signals

What are embryonic lethals?

mutations with phenotypes causing death at the embryonic/larval stage

• cannot be bred for study→ look for recessive mutations

What is a maternal effect gene?

When the mother is mutant, resulting in a mutant offspring, regardless of the offspring’s own genotype.

• Maternal effect genes control orientation of egg→egg-polarity genes

• set up anterior/posterior and dorsal-ventral axes of embryo

What is a bicoid?

Two-tailed gene

What are morphogens?

Gradients of substances establish an embryo’s axes and other features of its form

Where is the bicoid mRNA found?

Extreme anterior end of mature egg

• bicoid protein then diffuses from anterior→ posterior, resulting in gradient

How was bicoid research groundbreaking?

1. Led to an identification of a specific protein required for some of the earliest steps in pattern formation

2. Increased our understanding of the mother’s critical role in the initial phases of embryonic development

3. Gradient of morphogens can determine polarity and position

What are oncogenes?

Cancer-causing genes in certain types of viruses and genomes

What are proto-oncogenes?

Normal versions of cellular genes that code for normal function

What are the 4 ways a proto-oncogene becomes an oncogene?

1. Epigenetic changes

2. Translocations

3. Gene amplification

4. Point mutations

How do epigenetic changes alter chromatin form?

leads to abnormal chromatin condensation for cells that are found in tumors

• if a mutation in a gene for a chromatin-modifying enzyme leads to loosened chromatin, a proto-oncogene could be expressed at abnormally high levels

How does movement of DNA within the genome turn proto-oncogenes into oncogenes?

Chromosomes that have been broken and rejoined incorrectly (translocated)→ ends up near an active promoter, the transcription of an incorrect sequence may increase, creating an oncogene

How does amplification of a proto-oncogene increase likelihoood of oncogenes?

Amplification increases the number of copies in a cell through repeated gene duplication

how do point mutations in a control element/proto-oncogene increase the likelihood of oncogenes?

A point mutation could increase the expresssion of an oncogene or change the protein product (mutation in proto-oncogene), leading to oncogene production

What are tumor-suppressor genes?

Cells that inhibit cell division

What is the ras gene?

G protein that relays a signal from a growth factor receptor to synthesizes a protein→ stimulates the cell cycle

What is the difference between a normal cell cycle-stimulating pathway vs a mutant one?

mutation in Ras gene/any other pathway component will lead to excessive cell division and cancer

What is the p53 gene?

tumor-suppressor gene that promotes the synthesis of cell cycle-inhibiting proteins

What are the specific roles of the p53 gene?

1. activates miRNAs that inhibit cell cycle

2. turns on genes directly involved in DNA repair

3. activates suicide genes that conduct apoptosis

What is the process of colorectal cancer?

1. Colon wall has the loss of tumor-suppressor gene APC

2. Grows into polyp, and activates the ras-oncogene→ loss off tumor-suppressor SMAD4

3. Grows into adenoma (large benign growth)→ loss of p53

4. Results in malignant cancer

What are the differences between mutant tumor-suppressant alleles and oncogene alleles?

Mutant tumor suppressant→ recessive

Oncogene→ dominant

What syndrome increases an individual’s lifetime risk to colon cancer?

Hereditary nonpoolyposis colon cancer (HNPCC)

What gene is commonly mutated in colorectal cancers?

APC gene

What genes are tumor-suppressors in breast tissue?

BRCA1: found in basal-like breast cancers

BRCA2: repairs breaks that occur in both strands of DNA