Cell Differentiation and Nucleus Structure in Cytology I

Cell

Basic structural and functional unit of any living thing

What are the 3 components of cell theory?

1. All living things are made of one or more cells

2. All cells come from existing cells

3. Cells are the basic building blocks of life

Characteristics of eukaryotic cells

Can be unicellular or multicellular

Form tissues

Have a nucleus

Have organelles

Characteristics of prokaryotic cells

Unicellular organisms that

Never form tissues

No nucleus

No organelles except ribosomes

What 2 components are found in prokaryotic but NOT eukaryotic cells?

Cell wall

Capsule

Cells are diverse in?

Structure and function

Human cells features are?

Eukaryotic cells that do not have a cell wall

Form human tissues

Undergo differentiation

Cell differentiation:

Interaction between genes and cellular microenvironment leading to different genetic expression.

What is the result of cell differentiation?

One or more functions become more important.

Two main outcomes:

1.Different appearance and structure

2.Functional specialization

Specific function(s) are supported by?

Specific function(s) supported by different morphology

Totipotent stem cell

A cell that can differentiate into any cell type, including all embryonic and extra-embryonic cell types

Differentiated cells

Are mature cells that have specialized functions supported by different morphology

What is the nucleus?

The command center of the cell, a membrane-bound organelle that stores genetic material and coordinates cell activities

What molecular machinery is involved in the nucleus?

Molecular machinery for:

DNA synthesis

RNAs processing

4 components of the nucleus?

1.Nuclear envelope

2. Nucleoplasm

3.Chromatin

4.Nucleolus

Nuclear envelope has?

Pores

Nucleoplasm is?

Fluid inside the nucleus

Nucleoplasm is AKA?

Karyoplasm

What does nucleoplasm contain?

water, dissolved ions, and a complex mixture of other molecules

Function of nucleoplasm?

suspension medium for other nuclear components

Chromatin contains?

DNA + proteins

Briefly explain the nuclear envelope

A selectively permeable barrier formed by two membranes with pores that separates nucleoplasm from cytoplasm

3 components of the nuclear envelope:

1. Inner membrane (associated with nuclear lamina)

2. Outer membrane (binds ribosomes)

3. Perinuclear space or cisterna (in between)

The outer membrane and perinuclear space of the nuclear envelope are continuous with?

with the rER

Structure of the nuclear pores:

Has various nucleoporins (core proteins) with eightfold symmetry around a lumen .

-> Nuclear pore complexes bridge inner and outer membranes.

Functions of the nuclear pores:

1. Passage of ions and small molecules by simple diffusion

2. Regulate movement of macromolecules between nucleoplasm and cytoplasm

How does movement occur in the nuclear pores?

GTP hydrolysis by GTPases provide energy for import and export

What does the nuclear lamina stabilize?

The nuclear lamina stabilizes the nuclear envelope

Describe the structure of the nuclear lamina

Meshwork of proteins closely associated with inner membrane

What are lamins?

intermediate filament proteins (lamins) bound to membrane proteins and associated with chromatin in nondividing cell

How many chromosomes are in human chromatin?

46 chromosomes (23 pairs)

Except gametes

How long is human DNA?

2 meters long

3.2 billion bp

What are histones?

Basic proteins (+)

Histones and DNA form structural units of DNA called

nucleosomes

Nucleosome structure

A core of eight histones and approximately 150 base pairs of DNA.

What does H1 histone do?

Histone associated with the wrapped DNA at the surface of the core

What are the other 8 histones?

H2A, H2B, H3, and H4

What is linker DNA?

50 - 80 bp that separates nucleosomes

How are nucleosomes key to DNA replication and transcription?

through histone modification and rearrangement

Chromatin folding

process where DNA and histones form nucleosomes, which further coil and condense into thicker structures

What is the thickest part of chromatin folding?

Larger loops of coiled DNA (300 nm thick)

(Some are unstable showing transcriptional activity)

How are larger loops tethered?

Many loops are tethered to condensins (protein complexes). ->Leads to chromatin condensation.

Describe heterochromatin

Transcriptionally inactive or less active.

Appearing in intensely basophilic clumps, and coarse, electron-dense material.

Where does heterochromatin dominate?

Predominates in cells with little or no metabolic activity

Describe euchromatin

Transcriptionally active and appears as dispersed fine granules with slightly basophilic clumps.

Where does euchromatin dominate?

Predominates in metabolically active cells

Chromosomes

Structures that contain maximum coiling of DNA, visible during mitosis and meiosis.

Somatic cells are diploid or haploid?

Somatic cells are diploid –> 2n

•22 pairs of autosomes and 1 pair of sex chromosomes

Gametes cells are diploid or haploid

•Gametes are haploid –> n

•22 autosomes and 1 sex chromosome

Cellular microenvironment

The local environment surrounding a cell that influences its behavior and differentiation

Karyotype

The number and appearance of chromosomes in the nucleus of a somatic cell.

Nucleolus

A highly basophilic region within the nucleus, prominent in protein-synthesizing cells, involved in ribosome production.

The nucleolus has a high concentration of what?

dense concentration of rRNA

Important function of the nucleolus?

Ribosomes production

•Transcription of molecules of rRNA

•rRNAs associate with ribosomal proteins

•Assembly of ribosomal subunits

What is the cell cycle?

The sequence of events that lead to cell division, consisting of four phases: Mitosis, G1, S, and G2.

Mitosis

A phase of the cell cycle where the cell divides to produce two identical daughter cells (clones)

Somatic cells undergo?

Mitosis:

Parent cell divides

two identical daughter cells (clones)

same DNA

G1 Phase

Usually the longest phase. RNA and protein synthesis.

S Phase

DNA replication; Histone synthesis; Beginning of centrosome duplication

G2 Phase

Relatively short; Accumulation of proteins. Prep for mitosis

G0 Phase

Quiescent state; Cellular differentiation; Cell cycle suspended (may or not restart)

Prophase of Mitosis

Nucleolus disappears;

Chromatin condenses;

Mitotic spindle forms -> Centrosomes migrate to opposed poles

Nuclear envelope vanishes

What happens as the nuclear envelope vanishes?

Phosphorylation of lamins and inner nuclear membrane protein

-> Nuclear lamina and nuclear pores disassemble

Metaphase

Chromosomes further condense;

Kinetochore proteins attach to chromosome center; Chromosomes align in the equator of the cell

Anaphase

Sister chromatids separate;

Migrate to opposite poles

During anaphase each sister chromatid is now what?

each sister chromatid is now a single chromosome

Telophase

Chromosomes back to uncondensed state;

Nuclear envelope reassembles around each set of daughter chromosomes

Where are the 2 pairs of chromosomes during telophase?

1 at each spindle (pole)

Why are actin and myosin important in telophase?

in telophase, actin and myosin filaments form a belt at the equator

Cytokinesis leads to

Constriction of ring forms cleavage furrow; Two daughter cells result

Stem Cells

Undifferentiated cells;

Divide infrequently;

One remains as stem cell, one becomes a progenitor cell

Progenitor Cells

Also known as transit amplifying cells;

Frequent mitosis to increase the number of new cells

Terminally Differentiated Cells

Low or no potential for cell division;

Renewal depends on stem cells

example of cells in G0 that may reenter the cell cycle

hepatocytes

Meiosis

Specialized process with two unique cell divisions;

Only in germ cells

What are germ cells?

Cells that will become sperm or oocytes – (gametes)

Characteristics of Meiosis

1.Crossovers between homologous chromosomes

2.Formation of four haploid cells

(each cell with only one chromosome)

Genetic Variability

New combinations of genes resulting from crossovers between homologous chromosomes

Meiosis I

Includes Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis

Prophase I

Longer than in mitosis;

Homologous chromosomes form synapsis - tetrads

Crossover occurs

Crossover is:

Reciprocal DNA exchanges during Prophase I that lead to genetic recombination

Metaphase I

•Random arrangement of homologous chromosomes

Two chromosome arrangements

Four genetic arrangements

Anaphase I

Homologous chromosomes separate

Migrate to opposed poles

Telophase I and Cytokinesis

Same as in mitosis

Result of meiosis I

1. two haploid cells (n)

2. Diploid chromosomes (each chromosome retain the two sister chromatids)

Haploid Cells

Cells with only one chromosome from each pair

Diploid Chromosomes

Each chromosome retains the two sister chromatids after Meiosis I

Meiosis II

Same as mitosis;

Results in four haploid daughter cells with haploid chromosomes