How Cells Reproduce

Chapter 8

Understanding Cell Division

• What instructions are necessary for inheritance?

• How are those instructions duplicated for distribution into daughter cells?

• By what mechanisms are instructions parceled out to daughter cells?

Reproduction

• Parents produce a new generation of cells or multicelled individuals like themselves

• Parents must provide daughter cells with hereditary instructions, encoded in DNA, and enough metabolic machinery to start up their own operation

Division Mechanisms

Eukaryotic organisms

– Mitosis

– Meiosis

Prokaryotic organisms

– Prokaryotic fission

Roles of Mitosis

• Multicelled organisms

– Growth

– Cell replacement

• Some protistans, fungi, plants, animals

– Asexual reproduction

Meiosis

• Functions only in sexual reproduction

• Precedes the formation of gametes (sperm and eggs) or spores

Chromosome

• A DNA molecule & attached proteins

• Duplicated in preparation for mitosis

One chromosome (unduplicated)

One chromosome (duplicated)

one chromatid two sister chromatids

one chromatid

Sister Chromatids

• Each chromosome and its copy stay attached to each other as sister chromatids until late in the nuclear division process

• Attach at the centromere

Nucleosome

• A nucleosome consists of part of a DNA molecule looped twice around a core of histone proteins

Organization of Chromosomes

DNA and proteinsarranged as cylindrical fiber

DNA

histone

one nucleosome

Cell Cycle

• Cycle starts when a new cell forms

• During cycle, cell increases in mass and duplicates its chromosomes

• Cycle ends when the new cell divides

Interphase

• Usually longest part of the cycle

• Cell increases in mass

• Number of cytoplasmic components

doubles

• DNA is duplicated

Stages of Interphase

Control of the Cycle

• Once S begins, the cycle automatically

runs through G2 and mitosis

• The cycle has a built-in molecular brake

in G1

• Cancer involves a loss of control over

the cycle, malfunction of the “brakes”

Stopping the Cycle

• Some cells normally stop in interphase

– Neurons in human brain

– Arrested cells do not divide

• Adverse conditions can stop cycle

– Nutrient-deprived amoebas get stuck in

interphase

Chromosome Number

• Sum total of chromosomes in a cell

• Somatic cells– Chromosome number is diploid (2n)

– Two of each type of chromosome

• Gametes– Chromosome number is haploid (n)

– One of each chromosome type

Human Chromosome Number

• Diploid chromosome number (n) = 46

• Two sets of 23 chromosomes each– One set from father– One set from mother

• Mitosis produces cells with 46 chromosomes – two of each type

Lots of DNA

• Stretched out, the DNA from one human somatic cell would be more than two meters long

• A single line of DNA from a salamander cell would extend for ten meters

Stages of Mitosis

Prophase

Metaphase

Anaphase

Telophase

Bipolar Mitotic Spindle

• Consists of two distinct sets of

microtubules

– Each set extends from one of the

cell poles

– Two sets overlap at spindle equator

• Moves chromosomes during

mitosis

microtubule ofbipolar spindle

Mitosis • Period of nuclear division

• Usually followed by cytoplasmic division

• Four stages:

Prophase

Metaphase

Anaphase

Telophase

mitosis,cytoplasmic

division

a Two of the chromosomes(unduplicated) in a parent cell at interphase

b The same two chromosomes, nowduplicated, in thatcell at interphase, prior to mitosis

c Two chromosomes(unduplicated) in the parent cell’s daughtercells, which both startlife in interphase

Early Prophase - Mitosis Begins

Duplicated chromosomes begin to condense

Late Prophase

• New microtubules are assembled

• One centriole pair is moved toward opposite pole of spindle

• Nuclear envelope starts to break up

Transition to Metaphase

• Spindle forms

• Spindle microtubules become attached to the two sister chromatids of each chromosome

Metaphase

• All chromosomes are lined up at the spindle equator

• Chromosomes are maximally condensed

Anaphase

• Sister chromatids of each chromosome are pulled apart

• Once separated, each chromatid is a chromosome

Telophase

• Chromosomes decondense

• Two nuclear membranes form, one around each set of unduplicated chromosomes

Results of Mitosis

• Two daughter nuclei

• Each with same chromosome number as parent cell

• Chromosomes in unduplicated form

Cytoplasmic Division

• Usually occurs between late anaphase

and end of telophase

• Two mechanisms

– Cleavage (animals)

– Cell plate formation (plants)

Animal Cell Division

Cell Plate Formation

When Control Is Lost

• Growth and reproduction depend on controls over cell division

• Checkpoint proteins:– Growth factors invite transcription of genes that help

the body grow– Other proteins inhibit cell cycle changes, such as after

chromosomal DNA gets damaged

• When all checkpoint mechanisms for a particular process fail, a cell loses control over its replication cycle

• An example of this is cancer

Neoplasms

• Are abnormal masses of cells that lost controls over how they grow and divide– Benign – grow slowly and retain

surface recognition proteins that keep them in a home tissue (noncancerous)

– Malignant – grow and divide abnormally, disrupting surrounding tissues physically and metabolically (cancerous)

HeLa Cells

• Line of human cancer cells that can be grown in culture

• Descendents of tumor cells from a woman named Henrietta Lacks

• Lacks died at 31, but her cells continue to live and divide in labs around the world

Culturing Cells

• Growing cells in culture allows researchers to investigate processes and test treatments without danger to patients

• Most cells cannot be grown in culture