CH 8: The Cellular Basis of Reproduction and Inheritance

– Asexual reproduction

• Chromosomes are duplicated and cell divides

• One copy of each chromosome is placed in each cell

• Each “daughter” cell is genetically identical to the parent and the other daughter– Type of eukaryotic cellular division required:

mitosis

Methods of Reproduction

Advantage = fast and convenientDisadvantage = very little genetic variation

Sexual reproduction

• Offspring inherit DNA from both of their parents– Type of eukaryotic cellular division required:

meiosis

• Offspring can show great variation

– Advantage = lots of genetic variation

– Disadvantage = metabolically expensive

Methods of Reproduction

Prokaryotic cells reproduce asexually

by a type of cell division called binary fission

• The circular DNA molecule replicates to form 2 chromosomes

• The chromosome copies move apart• The cell elongates• The plasma membrane grows inward, dividing the

parent into two daughter cells

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Continued elongation of the cell and movement of copies

Duplication of chromosomeand separation of copies

Plasmamembrane

Cell wall

Prokaryoticchromosome

Division intotwo daughter cells

Eukaryotic Asexual Reproduction

• Mitosis:– Purpose:

• Asexual reproduction in single celled organisms• Growth and repair in multicelled organisms

– An exact copy of the cell’s DNA is made*, the copies separated, and each copy is put in a new cell.

• *Put another way…an exact copy of each chromosome is made

Mitosis

• Mitosis involves one cellular division.– 1 cell 2 cells (called daughter cells)

– Daughter cells are genetically identical– Chromosome number does not change.

Eukaryotic Chromosomecondensed form

– Sister chromatids have identical DNA

– Centromere • Kinetechore on

centromere provides binding site for microtubules

Sister chromatids

Eukaryote Chromosome Structure

Histone core is made up of 8 proteins

Histone core shown in greater detail, see page 212

A nucleosome is 2 wraps of DNA around a histone core

Cell Cycle

• Cell cycle describes the “life cycle” of a cell- Cell cycle is tightly controlled– G1– S Interphase– G2

Mitotic Phase– Mitosis

• Prophase, metaphase, anaphase, telophase

– Cytokinesis

Mitosis

– Mitosis = division of the cell’s DNA and nucleus in a eukaryotic cell

– Cytokinesis = division of the cytoplasm (cell)

– Mitosis occurs in somatic cells such as….

Cell Cycle

Cell Cycle

Interphase G 1 - period of cell growth

S - DNA synthesis • An exact copy is made of each chromosome• Copies are joined at the ________

G 2 – cell continues to grow and prepares to divide• e.g. centrioles duplicate in animal cells

G1 CheckpointChecks Cell Size, Organelles, Nutrition

M Checkpoint• Chromosomes Aligned?• Spindle Fibers Attached?

G2 Checkpoint• DNA Replicated?• Cell Division Machinery OK?

GO SIGNAL

Cell Completes Cell Cycle

STOP SIGNAL

Waits to Grow Larger

Control of the Cell Cycle - Checkpoints

Mitosis

• Mitosis (division of nucleus/chromosomes) follows interphase – see pages 130/131 – 4/5 phases

• Prophase,Prometaphase• Metaphase• Anaphase• Telophase (and cytokinesis)

Prophase

• Chromosomes condense, become visible under microscope

• Centriole pairs* move towards poles (animal only)

• Nucleoli disappear

• *centriole pair = centrosome

Prometaphase

• Transition from prophase to metaphase– Nuclear envelope breaks up and forms

vesicles– Microtubules arranged as spindle fibers attach

the kinetechore on the centromere of each sister chromatid to opposite poles

• Attach to centrioles in animal cells

Plant Prometaphase

• Prometaphase in a plant cell

• Chromosomes are visible

• Nuclear envelope is breaking down

• Spindle fibers cannot be seen in this micrograph.

Metaphase

• Spindle microtublules push and pull chromo to middle of cell

• Centromeres line up across the middle of the cell

• Microtubules running pole to pole elongate the cell– Not visible in this

micrograph

Animal Metaphase

Plant Metaphase

• Chromosomes tend to be “messier” in plant metaphase

Anaphase

• Sister chromatids separate at the centromere

• MT* pull sister chromatids to opposite poles

• MT continue to elongate cell– This also helps to

separate chromatids

* MT = microtubules

•Animal anaphase

Plant Anaphase

• Separated sister chromatids (daughter chromosomes) clearly visible

Telophase

• Telophase starts when chromatids reach poles

• Goal is to make 2 new nuclei– Chromo. unwind– Nucleoli reappear– Nuclear envelope

reforms from vesicles

• Animal cell shown

Cytokinesis

• Cytokinesis – division of cytoplasm

• Begins during telophase

• Different in plant and animal cells

Animal Cytokinesis

• Microfilaments wrap around the center of the cell and then contract

• Creates cleavage furrow

• Cell “squeezed” in 2

Page 132

Plant Cytokinesis

• Vesicles containing cell wall material line up across middle of cell

• Vesicles merge and form cell plate

• Cell plate grows until it divides the cell in 2

Cell plate

MITOSIS

2N

2N

2N

1. Interphase2. Prophase3. Metaphase4. Anaphase5. Telophase6. Cytokenesis

Is this a plant or an animal cell?

Mitosis Review

• Comparison Plant and Animal Mitosis

• Mitosis

• Animal Cell Mitosis

• Plant Cell Mitosis

Meiosis

• Meiosis is needed for sexual reproduction– Purpose of meiosis is to create gametes

• Egg and sperm in humans• Needed for sexual reproduction• Gametes have only one copy of each type of

chromosome

– Occurs in germ cells• Ovaries and testes of humans

Related Terms

• Diploid = 2 copies of each type of chromosome present (2N)

• One copy came from mom’s egg and the other from dad’s sperm

• Human diploid number = 46 (also say 2N = 46)

• Haploid = 1 copy of each type of chromosome present (N)

• Human haploid number = 23 (N = 23)• Gametes are haploid

Overview Meiosis

• Meiosis separates homologous chromosomes and produces cells with a single set of chromosomes– Homologous Chromosomes: pair of

chromosomes with genetic information about the same traits, page 136

Human KaryotypeHomologous Chromosomes

Meiosis

• The process of meiosis requires 2 cellular divisions – page 137

– One division to separate homologous chromosomes

– Second division to separate duplicated chromosomes

MEIOSIS

MEIOSIS I

Begins With:

•Duplicated Chromosomes

•Diploid (2N) Cells

Functions:

•Separate Homologous Chromosomes

•Go From Diploid (2N) to Haploid (N)

Meiosis 1

Homologous Pair

MEIOSIS II

Begins With:

•Duplicated Chromosomes

• Haploid (N) cells

Function:

•Separate Sister Chromatids•Creates gametes

Meiosis II

Crossing over occurs in meiosis I

2N = 2

2 cells, N = 1 for each

Sister chromatids separate in meiosis II

4 cells, N = 1 for each.Chromosomes are different due to crossing over

Homologous chromosomes separate in meiosis I

Meiosis I

• Prophase I – Cell is diploid – Chromosomes are duplicated

• Duplicated chromosomes form tetrads• Tetrad = pair of homologous chromosomes

– Crossing over occurs • Exchange of genetic material between

homologous chromosomes

CROSSING OVER

• During Prophase I

Exchange of genetic material between Homologous Chromosomes

Produces new genetic combinations--Chromosomes with both

Maternal & Paternal components

Meiosis 1

Meiosis 2

Gametes

M F

occurs at CHIASMA

Meiosis I

• Prophase I, continued

– Chromosomes condense (super-coil)– Centrioles move towards opposite poles

(animal only)– Spindle fibers begin to assemble – Nuclear envelope breaks down (always

signals end of a prophase )

Meiosis I

Metaphase I– Spindle fibers push

and pull the tetrads to the middle of the cell.

– Spindle fibers attach each chromosome of the pair to one pole

Meiosis I

• Anaphase I– Homologous

chromosomes are separated and pulled to opposite poles by the spindle fibers

– Microtubules running pole to pole lengthen and elongate the cell

Meiosis I

• Telophase I and Cytokinesis– Chromosomes reach

the poles – still duplicated

– Cell divides in two• Animal cells - cleavage

furrow squeezes cell in two

• Plant cells – cell plate divides cell in two

– Generally, the nucleus does not reform

At the end of Meiosis I

– Homologous chromosomes have been separated

• Chromosomes are still duplicated• Sister chromatids are no longer identical due to

crossing over

– Chromosome number has been cut in half (to haploid number)

• Count centromeres to count chromosomes

Crossing over occurs in meiosis I

2N = 2

2 cells, N = 1 for each

Sister chromatids separate in meiosis II

4 cells, N = 1 for each.Chromosomes are different due to crossing over

Homologous chromosomes separate in meiosis I

Meiosis II

• Prophase II – in each cell – Centriole pairs separate and move to opposite

poles (animal only)– Spindle fibers attach to kinetechore

(centromere) of each chromosome• Remember chromosomes are still duplicated• Notice that each chromo is attached to both poles

(as in mitosis)

Meiosis II

• Metaphase II– Spindle fibers push and pull duplicated

chromo. To the center of the cell

Metaphase II

METAPHASE I – tetrads line up across the center of the cell

METAPHASE II – duplicated chromosomes line up

Anaphase II

– Spindle fibers separate the sister chromatids

– One copy of each chromosome moves to each pole

– Microtubules running pole to pole lengthen and elongate the cell

Telophase II

• Telophase II and Cytokinesis– Nucleus reforms in

each cell (4 cells in total)

– Cytoplasm divides

• Meiosis web link

Meiosis

• Two cellular/nuclear divisions– 1st division separates homologous

chromosomes (each in its duplicated state)– 2nd division separates duplicated

chromosomes

• 1 cell with 2N chromo 2 cells with N duplicated chromo 4 cells with N chromo

Meiosis

• End result of meiosis– 4 cells are made– Each cell has the haploid number of chromo.

• One copy of each type of chromo

– No two gametes are identical due to:• independent assortment of homologous

chromosomes (page 141)• crossing over during meiosis I

Cell Division Summary

• Page 140 provides a summary of the 2 types of cellular division.

• Given a picture of a phase of mitosis or meiosis you should be able to:– Identify the phase and division type– Label as appropriate: spindle fibers,

centrioles, sister chromatids, homologous chromosomes, centromere/kinetechore, nuclear envelope…