Chapter 8: Cell Reproduction

Chromosome Structure Chromatin – made of DNA and protein Seen in a non-dividing cell Chromosomes – Chromatin that has coiled tightly Seen in a dividing cell Histones – proteins that help to coil the DNA and maintain the shape of chromosomes

After replication, there are 2 identical sister chromatids, held together by a centromere

Each species has a set number of chromosomes: Humans – 46 Crayfish – 200 Corn – 20 Adder’s tongue fern – 1262 Chimpanzee - 48 Sand dollar – 52 Dog – 78 Cat - 32

Viewing the Chromosomes Karyotype - a photograph of the chromosomes in a dividing cell Shows the gender of the child Shows abnormalities in chromosome number or structure. Humans have 23 pairs of homologous chromosomes (chromosomes with the same genes), or 46 chromosomes total. You get 1 of each chromosome from mom, and 1 from dad.

Of the 23 pairs of chromosomes: 1 pair are sex chromosomes – determine your gender; may determine other traits as well Females are XX Males are XY 22 pairs are autosomes – all other pairs of chromosomes; do NOT determine gender

How is karyotyping done? Cells are taken from the fetus of a pregnant woman Chemicals added to the cells make them divide Another chemical stops division at the midpoint The stained cells are photographed and can be paired based on cross-bands, size, and shape.

Human karyotype preparation

Normal male karyotype

Down syndrome karyotype

Body cells contain the diploid (2n) number of chromosomes – 2 chromosomes of each kind (1 from each parent) Sex cells (eggs and sperm) contain only 1 chromosome of each kind – haploid (1n) number of chromosomes

Why do cells divide – why can’t they just grow bigger? Surface area represents ability to take in/get rid of materials. Volume represents needs of the cell – food, oxygen, water, etc.

Small cube 1 mm tall Surface area = 6 mm 2 Volume = 1 mm 3 Surface area to volume ratio = 6:1 Larger cube 2 mm tall Surface area = 24 mm 2 Volume = 8 mm 3 Surface area to volume ratio = 3:1

As a cell grows, the volume of the inside grows faster than the cell membrane around the outside What happens when a cell gets too big? It could starve – can’t take in enough food to support needs It could poison itself – can’t get rid of waste fast enough So cells stay small for exchange of materials

Rather than grow bigger, cells divide and increase in number Cell division occurs when: Body growth Maintenance and repair Fighting infection Replacing worn/dead cells

The Cell Cycle An orderly sequence of events that occurs from the time a cell is first formed until it divides into two new cells.

Most of the cell cycle is spent in interphase (time between cell divisions): G 1 stage – cell growth and activity S stage – DNA replication occurs (S stands for synthesis of DNA) G 2 stage – cell prepares for cell division

Amount of time spent in interphase varies – average for adult mammals is 20 hours Nerve cells and muscle cells exit the cell cycle  G 0 phase Body never makes more of them

Following interphase is the M stage, including mitosis and cytokinesis Mitosis: 2 daughter cells are produced that are identical to the Occurs in body cells Diploid cells produce other diploid cells Takes about 4 hours ell Division/Mitosis/Introduction_to_Mitosis.asf

Mitosis overview

1.Centriole 2.Chromatin 3.Nucleolus (in yellow) 4.Nuclear membrane 5.Spindle fibers 6.Chromosome (replicated) 7.Centromere 8.Sister Chromatids (each half of replicated chromosomes) – become daughter chromosomes 9.Cell membrane 10. Cell membrane pinches in

Early Prophase Chromatin coils to become chromosomes Centrioles move to opposite ends of the cell Nuclear membrane and nucleolus disappear

Late Prophase Spindle fibers form Chromosomes become attached to spindle fibers – centromere attaches to spindle fibers

Metaphase Chromosomes line up at equator (middle)

Anaphase Sister chromatids separate, becoming daughter chromosomes, and move toward opposite ends of cell

Telophase Spindle fibers disappear Nuclear membrane and nucleolus reappear Chromosomes uncoil and turn back into chromatin

Cytokinesis, or division of cytoplasm, accompanies mitosis. Division of the cytoplasm begins in anaphase, but is not completed until end of telophase. In animal cell, cell membrane is flexible and pinches in to divide cell. In plant cells, rigid cell wall does not bend, so a cell plate forms to divide the cell.

Mitosis in Plant Cells Same phases as in animal cells No centrioles Cell plate forms instead of cell membrane pinching in

Cytokinesis in animal cells /Cell Division/Mitosis/Mitosis_Video_Quiz.asf

Cytokinesis in plant cells

Cell Division in Prokaryotes The process of asexual reproduction in prokaryotes is called binary fission. The two daughter cells are identical to the original parent cell, each with a single chromosome. Following DNA replication, the two resulting chromosomes separate as the cell elongates. Cell divides without cell structures seen in plants & animals

Meiosis Produces sex cells (gametes) Female gamete – egg Male gamete - sperm Reduces the chromosome number Egg or sperm cells each have only one of each kind of chromosome Diploid cells (2n)  haploid cells (1n)

Overview of meiosis

Meiosis involves two cell divisions (meiosis I and II) Produces four haploid cells. Fertilization (fusion of egg and sperm) - restores the diploid number of chromosomes

Meiosis I Original cell produces 2 new cells Each new cell has 1 chromosome from each homologous pair Each new cell is haploid (1n)

Meiosis II Each of the 2 cells made in meiosis I splits Result is 4 haploid cells Chromatids of chromosomes are separated Errors in meiosis can result in missing or extra chromosomes

Meiosis in males vs. females: Males: Occurs in testes Spermatogenesis Makes 4 haploid sperm Females: Occurs in ovaries Oogenesis Makes 1 large egg & 3 polar bodies (that die) – cells divide unevenly

Spermatogenesis

Oogenesis

Meiosis vs. Mitosis Mitosis Body cells Diploid cells make diploid cells End result 2 cells No variation in cells produced Meiosis Sex cells Diploid cells make haploid cells End result up to 4 cells Cells made are different from parent

Meiosis compared to mitosis

Sexual vs. Asexual Reproduction Sexual: 2 parents Offspring are genetically different from parent – mix of traits Union of egg & sperm Asexual: 1 parent Offspring are identical to parent Binary fission in bacteria Regeneration in worms

Cloning: Produces an organism that is genetically identical to the parent Steps: 1.DNA is removed from a fertilized egg 2.Fertilized egg cell is fused with cell of individual being cloned 3.Egg with new DNA is implanted into a woman