Lesson 4 learning objectives

Nucleus

The organelle that contains most of the cell's DNA and controls gene expression.

Nuclear envelope

A double membrane surrounding the nucleus that separates nuclear contents from the cytoplasm.

Nuclear pore

A protein-lined opening through the nuclear envelope that regulates movement of substances between the nucleus and cytoplasm.

Nuclear pore complex

A large protein structure forming a passageway through the nuclear envelope.

Nuclear envelope outer membrane

The outer nuclear membrane is continuous with the rough endoplasmic reticulum.

Nuclear envelope inner membrane

The inner nuclear membrane faces the nucleoplasm and helps organize the nucleus.

Nucleoplasm

The fluid inside the nucleus.

Nucleolus

A dense region within the nucleus where ribosomal RNA is produced and ribosomal subunits are assembled.

Chromatin

DNA associated with proteins in the nucleus.

Chromosome

A single long DNA molecule associated with proteins.

Histone protein

A protein around which DNA is wrapped to help organize and package DNA.

DNA packaging

DNA is wrapped around histone proteins and further condensed to form chromosomes.

Nuclear lamina

A protein network lining the inner nuclear envelope that helps support nuclear shape.

Nuclear transport

Movement of molecules between the nucleus and cytoplasm through nuclear pores.

What enters the nucleus

Proteins and other molecules needed for nuclear functions can enter through nuclear pores.

What leaves the nucleus

RNA molecules and ribosomal subunits can leave the nucleus through nuclear pores.

Gene

A segment of DNA that contains instructions for making a functional product, such as a protein or RNA.

Genome

The complete set of genetic information in a cell or organism.

DNA

Deoxyribonucleic acid, the molecule that stores hereditary information.

DNA function

DNA stores genetic information and provides instructions for cellular products.

DNA nucleotide

A DNA building block consisting of a sugar, phosphate group, and nitrogenous base.

DNA sugar

Deoxyribose.

DNA bases

Adenine, thymine, cytosine, and guanine.

Purine

A nitrogenous base with a double-ring structure; adenine and guanine are purines.

Pyrimidine

A nitrogenous base with a single-ring structure; cytosine and thymine are pyrimidines.

Adenine (A)

A purine base that pairs with thymine in DNA.

Thymine (T)

A pyrimidine base that pairs with adenine in DNA.

Cytosine (C)

A pyrimidine base that pairs with guanine in DNA.

Guanine (G)

A purine base that pairs with cytosine in DNA.

DNA base-pairing

Adenine pairs with thymine, while cytosine pairs with guanine.

Hydrogen bonds in DNA base pairs

A-T pairs form two hydrogen bonds; C-G pairs form three hydrogen bonds.

DNA strands

DNA consists of two nucleotide strands held together by hydrogen bonds between complementary bases.

DNA double helix

The twisted, ladder-like structure formed by the two DNA strands.

Complementary base sequence

The sequence of one DNA strand determines the sequence of the other because bases pair specifically.

RNA

Ribonucleic acid, a nucleic acid involved in using genetic information to make proteins and perform other cellular functions.

RNA sugar

Ribose.

RNA bases

Adenine, uracil, cytosine, and guanine.

RNA vs DNA

RNA contains ribose and uracil and is usually single-stranded; DNA contains deoxyribose and thymine and is double-stranded.

Messenger RNA (mRNA)

RNA that carries genetic instructions from DNA to ribosomes for protein synthesis.

Transfer RNA (tRNA)

RNA that carries amino acids to ribosomes during protein synthesis.

Ribosomal RNA (rRNA)

RNA that combines with proteins to form ribosomes and participates in protein synthesis.

Transcription

The process of using DNA as a template to produce RNA.

Transcription location

Transcription occurs in the nucleus.

Translation

The process of using an mRNA sequence to assemble a protein.

Translation location

Translation occurs at ribosomes in the cytoplasm or on the rough ER.

Central dogma

Genetic information generally flows from DNA → RNA → protein.

Genetic code

The rules by which nucleotide sequences in mRNA specify amino acids.

Codon

A sequence of three nucleotides in mRNA that specifies an amino acid or a stop signal.

Start codon

The codon AUG, which signals the beginning of translation and codes for methionine.

Stop codons

UAA, UAG, and UGA, which signal termination of translation.

Anticodon

A three-nucleotide sequence on tRNA that is complementary to an mRNA codon.

Protein synthesis

The process by which cells use genetic information to assemble proteins through transcription and translation.

Protein assembly

Ribosomes link amino acids together in the order specified by mRNA.

Polypeptide

A chain of amino acids produced during protein synthesis.

Peptide bond

The covalent bond linking amino acids together in a protein or polypeptide.

Protein folding

The polypeptide chain folds into a specific three-dimensional shape required for its function.

Protein processing

Modification, folding, and targeting of newly synthesized proteins so they can perform their functions.

Post-translational modification

Changes made to a protein after translation that can affect its structure, activity, or location.

Protein targeting

The process of directing a newly synthesized protein to its proper cellular destination.

Signal sequence

A sequence of amino acids that directs a protein to a particular cellular location.

Signal recognition particle (SRP)

A particle that recognizes certain signal sequences and directs the ribosome to the rough ER.

Rough ER protein targeting

Proteins with appropriate signal sequences can be synthesized into or across the rough ER membrane.

Secretory protein

A protein synthesized on the rough ER and ultimately released from the cell by exocytosis.

Membrane protein

A protein synthesized on the rough ER that becomes incorporated into a cellular membrane.

Golgi protein processing

The Golgi modifies, sorts, and packages proteins received from the rough ER.

Protein assembly location

Protein synthesis occurs on ribosomes.

Protein synthesis on free ribosomes

Free ribosomes generally synthesize proteins that function in the cytosol or certain internal organelles.

Protein synthesis on bound ribosomes

Ribosomes attached to rough ER generally synthesize proteins destined for secretion, membranes, or certain organelles in the endomembrane system.

Gene regulation

Control of when, where, and how much a gene is expressed.

Gene expression

The process by which information in a gene is used to produce a functional RNA or protein.

Why cells regulate genes

Different cells express different genes, allowing cells to develop specialized structures and functions.

Differential gene expression

Different cell types express different sets of genes even though they generally contain the same DNA.

Example of differential gene expression

Muscle cells and nerve cells have the same genome but express different genes, producing different structures and functions.

Regulation before transcription

Cells can control whether transcription occurs.

Regulation during transcription

Cells can regulate how efficiently RNA is produced from DNA.

Regulation after transcription

Cells can modify, degrade, or control the use of RNA before it is translated.

Regulation during translation

Cells can control whether and how efficiently an mRNA is translated into protein.

Regulation after translation

Cells can modify, activate, deactivate, or degrade proteins.

DNA replication

The process of copying DNA before cell division.

Purpose of DNA replication

To produce a complete copy of genetic information for daughter cells.

Semiconservative replication

Each new DNA molecule contains one original strand and one newly synthesized strand.

DNA replication timing

DNA replication occurs before cell division during the S phase of interphase.

Helicase

An enzyme that separates the two DNA strands by breaking hydrogen bonds between complementary bases.

Replication fork

The Y-shaped region where DNA strands are separated and new DNA is synthesized.

DNA polymerase

An enzyme that adds nucleotides to a growing DNA strand using the original strand as a template.

Complementary DNA synthesis

New DNA nucleotides are added according to complementary base-pairing rules.

DNA ligase

An enzyme that joins DNA fragments together.

Okazaki fragments

Short DNA fragments produced on the lagging strand during DNA replication.

Leading strand

The DNA strand synthesized continuously during replication.

Lagging strand

The DNA strand synthesized discontinuously as Okazaki fragments.

Why the lagging strand is discontinuous

DNA polymerase can synthesize DNA only in one direction, requiring the lagging strand to be built in separate fragments.

DNA replication sequence

Helicase separates DNA strands; DNA polymerase adds complementary nucleotides; DNA ligase joins fragments on the lagging strand.

Cell cycle

The sequence of events through which a cell grows, replicates its DNA, and divides.

Interphase

The portion of the cell cycle during which the cell grows, performs normal functions, and replicates DNA.

G1 phase

The first growth phase of interphase when the cell grows and performs normal functions.

S phase

The phase of interphase during which DNA is replicated.

G2 phase

The second growth phase of interphase when the cell prepares for division.

M phase

The phase of the cell cycle involving mitosis and cytokinesis.

Mitosis

Nuclear division that produces two genetically similar daughter nuclei.

Cytokinesis

Division of the cytoplasm into two daughter cells.