Lecture 15 - Protein Synthesis, Mutations, & Regulation of Gene Expression

BIO Flashcards

Form of specific sequences of DNA nucleotides

“information content” of GENES is…

Synthesis of proteins and of RNA molecules invovled in protein synthesis

Specific sequence of DNA nucleotides codes for…

Allele

Variation in the nucleotide sequence of a specific gene

Variation in the proteins synthesized

Nucleotide variances among alleles translates into ____

Traits, features, biochemistry, and physiology among different individuals within the same and across different species

Variation proteins leads to different _______

Proteins

Link between genotype and phenotype

Gene expression

Process by which DNA directs protein synthesis

Genes

Instructions for making specific proteins

mRNA

Bridge between DNA and protein synthesis

1. Backbone sugar is ribose (not deoxyribose)

2. Uracil replaces thymine

3. Single strand of nucleotides (vs double helix)

Differences between RNA and DNA

1. Transcription

2. RNA processing

3. Translation

Three major stages of DNA to proteins

Transcription

DNA-direct synthesis of mRNA

RNA Polymerase

Separates DNA strands and joins RNA nucleotides complementary to the DNA template strand, only works 5’ —> 3’, do NOT need a primer

Promoter

Where RNA attaches and begins transcription

Terminator

Sequence that signals the end of transcription

Transcription in Eukaryotic Cells

Occurs in nucleus of cells, results in pre-mRNA

Primary Transcript

Initial RNA transcript, further processing yields mRNA

RNA Processing

When eukaryotic cells modify RNA after transcription… occurs in nucleus.

1. Both ends of primary transcript are changed

2. Specific interior sequences are cut out and removed; remaining sequences are spliced back together

Head and Tail Modifications

1. Facilitate the export of mRNA from the nucleus

2. Help protect mRNA from hydrolytic enzymes

3. Help the ribosomes attach to the 5’ cap of the mRNA

   Examples: 5’cap & poly-A tail

5’ cap

At 5’ end of pre-mRNA, modified form of guanine is added

Type of head/tail modification

Poly-A Tail

At 3’ end, enzyme adds 50-250 adenine nucleotides

RNA splicing

Removes introns and joins exons to create a shortened mRNA molecule with a continuous coding sequence

Introns

Noncoding segment of nucleotides that lie between coding regions

Exon

Region of mRNA that is eventually expressed… meaning they are translated into amino acid sequences

Translation

RNA-directed synthesis of a polypeptide

transfer RNA (tRNA)

Transfers amino acids from the “cytoplasmic pool” to a growing polypeptide chain on a ribosome, bears one specific amino acid at one end on the other has an anticodon

Codon

Sequence of three adjacent nucleotide bases within a strand of mRNA that provides the genetic code for a particular amino acid

Anticodon

Group of three bases on a tRNA molecule that are complementary to an mRNA codon

Peptide Bonds

Ribosomes join the amino acids into a chain via _____ bonds

Ribosomal RNA (rRNA)

Combine with proteins to form the large and small subunits of the ribosome, in eukaryotic cells the formation of these subunits occurs in the nucleolus. Catalyzes the formation of the polypeptide bond

P site

Found in a ribosome, holds the tRNA carrying the growing polypeptide chain

A site

Found in a ribosome, Holds the tRNA carrying the next amino acid to be added to the chain

E Site

Part of ribosome, where discharged tRNA leaves the ribosome

Occurs at the carboxyl end of polypeptide

Ribosomes attach each new amino acid with a peptide bond on which end of the polypeptide?

Polyribosomes or Polysomes

Multiple ribosomes simultaneously translating a single mRNA strand to make multiple copies of polypeptide

Chaperonin

Is a protein that helps the polypeptide fold correctly… primary structure determines secondary and tertiary structure

During and After synthesis

When does polypeptide folding occur?

Post-translational Modification

Addition of sugar, lipids, or phosphate groups to polypeptide to make it functional

Quaternary Structure

2 or more polypeptides join to form a protein with a ______ structure.

Central Dogma

Francis Crick named the concept of DNA transcribed to RNA which is translated to Protein the _____.

Mutations

Changes in the genetic material of a cell, ultimate source of new alleles/genes, if it has an adverse effect on phenotype, it is referred to as a genetic disorder/hereditary disease

Large-scale mutations

Long segments of DNA are affected by translocations, duplications, or inversions

Point Mutations

Chemical changes in just one nucleotide pair of a gene, if occurs in gamete may be transmitted to future generations

Sickle-cell disease

Caused by a mutation of a single nucleotide pair in the gene (point mutation), genetic disorder.. leads to an abnormal protein

Nucleotide-Pair substitution

Replacement of one nucleotide and its partner with another pair of nucleotides, some have no effect on protein function (due to redundancy of genetic code)

Silent Mutation

A change in nucleotide pair transforms one codon into another that is translated into the same amino acid

Missense Mutation

A change in nucleotide pair transforms one amino acid for another with little effect on protein function

Nonsense Mutations

A change of an amino acid codon into a stop codon, causing early termination of translation leading to a nonfunctional protein

Insertions and Deletions

Additions or losses of nucleotide pairs in a gene, more likely than substitution to have disastrous effect on protein made

Frameshift Mutation

All nucleotides downstream of a deletion or insertion become improperly grouped into codons, resulting in extensive missense

DNA replication, DNA repair, or DNA recombination

When do mutations occur?

Mutagens

Chemical or physical agents that interact with DNA to cause mutations

radiation like X-ray, ultraviolet light

Forms of Physical agents

Chemical Mutagens

Cause Mutations in different ways… nucleotide analogs that may be subbed into DNA, pair incorrectly during DNA replication OR interfere with DNA replication by inserting into DNA OR cause chemical changes in bases changing pairing properties

Gene

Region of DNA that can be expressed to produce a final functional product that is either a polypeptide or a protein

By transcription into RNA and then translation into a polypeptide that performs a specific function

How are genes expressed?

Proteins

What brings about an organism’s observable phenotype?

Cell only expresses a small subset of its genes, gene expression is precisely regulated

A typical cell expresses?

Changes in environmental conditions

Both prokaryotic and eukaryotic cells alter their patterns of gene expression in response to…

Each cell contains the same, complete set of genes but expresses a different subset of genes

Can modulate protein making to match demand

Multicellular eukaryotes develop and maintain multiple cell types (Cell diversity)

In bacteria it is often regulated at the transcription stage… Eukaryotes regulate at various stages

When is gene expression regulated?

Cancer or other disorders

Disruptions in gene regulation may lead to _____.

Bacteria that express only the genes whose products are needed by the cell.

Ex: bacterium in tryptophan-rich environment stops producing tryptophan to conserve its resources

Natural selection favors bacteria that ____.

Feedback Inhibition

The pathway’s end product blocks the activity of the first enzyme in the pathway.
Typical of anabolic pathways.

Allows cell to adapt to short-term fluctuations in supply of a needed substance

Operon Model - Operator, repressor, and operon

Basic mechanism for control of gene expression in bacteria… regulation of one gene by another that operates as a “master” switch. Control of enzyme production occurs at transcription.

Differential gene expression OR Cell Differentiation

Expression of different genes by cells with the same genome. All cells in organism have identical genome, but subset of genes expressed in cell of each type is unique.

20%

Typical human cell expresses about ____ of its genes at any given time.

Imbalances and diseases like cancer

Problems with gene expression and control can lead to ______.

True

True or False… Genes of densely condensed heterochromatin are usually not expressed.

N-terminus

Protrudes outwards from the nucleosome, found on each histone molecule

Histone Acetylation

Addition of an acetyl group called ______ promotes transcription… or deacetylation of lysines in histone tails play a direct role in regulation of gene transcription

Attachment of methyl group (methylation)

Addition of ____ to histone tail leads to condensation of chromatin (i.e. no transcription)

Addition of phosphate group

Called Phosphorylation… added to an amino acid next to a methylated amino acid leads to decondensation of chromatin

DNA methylation

Reduces gene expression. Can occur at tails of histone proteins or on certain bases in DNA, usually cytosine

Occurs in most plants, animals and fungi.

Occurs more on inactive DNA than on actively transcribed regions

Removal of extra methyl groups can turn on some of these genes.

Individual genes are usually more heavily methylated in cells where they are not expressed. How to turn on gene?

Deficient DNA methylation leads to abnormal embryonic development. Once methylated, genes usually stay that way through successful cell divisions.

DNA methylation causes the long-term inactivation of genes during cellular differentiation

Control elements

Noncoding DNA segments that serve as binding sites for protein transcription factors

Transcription Factors

Collection of proteins that mediate the binding of RNA polymerase and onset of transcription

Enhancers

Distal control elements… binds to activator proteins which turn on transcription. Composed of about ten control elements, which can only bind to one or two specific transcription factors

Binding of specific transcription factors, either activators or repressors, to control elements of enhancers

Gene expression in Eukaryotic cells can be altered by…

Repressors

Inhibit gene expression by blocking the binding of activators to their control element

Can bind directly to control-element DNA, turning off transcription even in the presence of activators

Alternative RNA splicing

Different mRNA molecules are produced from the same primary transcript, different RNA segments are treated as exons/introns.

Increases number of possible human proteins.

75% - 100% of human genes probably undergo this process

Regulatory Proteins

Control intron-exon choices by binding to regulators sequence within primary transcript, specific to cell type