Gene Expression and Nucleus

Genes

• Hereditary information is transmitted in the form of distinct units

• Consist of DNA that codes for functional products that are usually proteins

Transcription

RNA is synthesized in an enzymatic reaction that copies information from DNA

Translation

the base sequence of RNA is used to direct the synthesis of a polypeptide

Regulation

an important aspect of almost every process in nature, especially gene expression

Selective gene expression

allows cells to be efficient, synthesizing only what is needed for each cell type

Gene expression is regulated at 5 main levels

1. Genome

2. Transcription

3. RNA processing and export

4. Translation

5. Posttranslational events

Genomic equivalence

almost all cells retain the same complete set of genes

Types of regulatory changes in genomic control

• Gene amplification (multiple copes of the same gene)

• Gene deletion (cells delete genes that aren’t required)

• DNA rearrangement (movements of DNA segments from one location to another)

• Chromatin decondensation (regulated by histone modifications and chromatin remodeling factors)

• DNA methylation (inactive regions of the genome)

Transcriptional control

• Different proteins produced by two cell types in the same individual result from differential gene transcription

• Controlled by regulatory transcription factors that bind DNA control sequences

RNA splicing

Allows cells to create a variety of different mRNAs from the same pre-mRNA

Alternative splicing

permits some splice sites to be skipped and others activated

Translational control

• regulate their rate of translation

• Some mechanisms work by altering ribosomes or protein synthesis factors

• Others work by regulating the activity/stability of the mRNA

Posttranslational control mechanisms

1. Structural alterations that influence protein function

2. Guiding of protein folding

3. Targeting to specific locations

4. Interaction with regulatory molecules

Ubiquitin

small protein containing 76 amino acids, which targets proteins for degradation by proteasomes

Proteasomes

large protein-degrading structures and the predominant proteases of the cytosol; they bind ubiquitin-labeled proteins and remove the ubiquitin; also play a role in eliminating defective proteins from cells

Nucleus

• the site within the eukaryotic cell where the chromosomes are localized and replicated and the DNA they contain is transcribed

• Most prominent feature of the cell

Outer membrane

continuous with the ER and contains proteins that bind actin and intermediate filaments (IFs) of the cytoskeleton

Nuclear pores

• specialized channels in the nuclear envelope where inner and outer membranes are fused

• Provide direct contact between the cytosol and the nucleoplasm

• Lines with a protein structure (nuclear pore complex)

Central granule

called the transporter and is likely involved in moving molecules across the nuclear envelope

Molecules that enter the nucleus through nuclear pores

Enzymes and proteins needed in the nucleus must be imported from the cytoplasm

Molecules that exit the nucleus through nuclear pores

RNAs that need to be translated and components of ribosomes must be exported from the nucleus

Nuclear localization signals (NLS)

• enable the protein to be recognized and transported by the nuclear pore complex

• usually 8–30 amino acids in length and often contains proline and the basic amino acids lysine and arginine