Cell Reproduction
Cell Division:
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All cells are derived from preexisting cells (Cell Theory)
Cell division is the process by which cells produce new cells
Cell division differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants,
& animals)
Some tissues must be repaired often such as the lining of gut, white blood cells, skin
cells with a short lifespan
Other cells do not divide at all after birth such as muscle & nerve
Why Cells Divide:
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Cells grow, but must remain small
To repair and replace cell parts
Maintain species population
Saving Genetic Information (DNA Replication):
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Each new cell must get a complete set of the DNA molecules
Requires that the DNA be copied before cell division
Structure of Chromosomes:
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DNA, deoxyribose nucleic acid, is a long thin molecule that stores genetic
information
DNA in a human cell is estimated to consist of six billion pairs of nucleotides
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Chromosomes are made of a DNA molecule tightly coiled around a protein called
Histones, visible only when the cell divides. When cell is not dividing DNA is not
visible and is uncoiled, thin, thread-like material called Chromatin
When chromosomes are duplicated before dividing, each chromosome in a pair is
called a sister chromatid
Centromeres hold duplicated chromosomes together before they are separated in
mitosis
Telomeres are the ends of chromosomes which are important in cell aging
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DNA in prokaryotes (bacteria) is one, circular chromosome
Number of Chromosomes in Cells:
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Humans have 23 pairs of chromosomes (diploid or 2n number) or 46 individual
chromosomes (23 from father & 23 from mother)
The 2 chromatids of a chromosome pair are considered homologous (have genes for
the same trait at the same location)
Homologs
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Every organism has a specific chromosome number
Organism
Bat
Alligator
Dog
Carrot
Chromosome #
44
32
78
18
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Fertilization, joining of the egg & sperm, restores the diploid chromosome number in
the zygote (fertilized egg cell)
Sex chromosomes, either X or Y, determine the sex of the organism
Two X chromosomes, XX, will be female and XY will be male
All other chromosomes, except X & Y, are called autosomes
Chromosomes from a cell may be arranged in pairs by size starting with the longest
pair and ending with the sex chromosomes to make a karyotype
A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23
total)
Human Male Karyotype
Genes:
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A section of DNA which codes for a protein is called a gene
Each gene codes for one protein
Humans have approximately 50,000 genes or 2000 per chromosome
About 95% of the DNA in chromosome is "junk" that does not code for any proteins
Cell Cycle:
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Cells go through phases or a cell cycle during their life before they divide to form
new cells
The cell cycle includes 2 main parts --- interphase, and cell division
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Cell division includes mitosis (nuclear division) and cytokinesis (division of the
cytoplasm)
Interphase is the longest part of a cell's life cycle and is called the "resting
stage" because the cell isn't dividing
Cells grow, develop, & carry on all their normal metabolic functions during
interphase
Interphase consists of 3 parts --- G1, S, & G2phases
Interphase:
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G1 or 1st Growth Phase occurs after a cell has undergone cell division
Cells mature & increase in size by making more cytoplasm & organelles while
carrying normal metabolic activities in G1
S or Synthesis Phase follows G1 and the genetic material of the cell (DNA) is
copied or replicated
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G2 or 2nd Growth Phase occurs after S Phase and the cell makes all the structures
needed to divide
Cell division in Prokaryotes:
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Prokaryotes such as bacteria do not have a nucleus
Prokaryotes divide into two identical new cells by the process of binary fission
Binary fission is an asexual method of reproduction
In binary fission, the chromosome, attached to cell membrane, makes a copy of itself
and the cell grows to about twice its normal size
Next, a cell wall forms between the chromosomes & the parent cell splits into 2
new identical daughter cells (clones)
Cell Division in Eukaryotes:
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Eukaryotes have a nucleus & membrane-bound organelles which must be copied
exactly so the 2 new cells formed from division will be exactly alike
The original parent cell & 2 new daughter cells must have identical chromosomes
DNA is copied in the S phase of the cell cycle & organelles, found in the
cytoplasm, are copied in the Growth phases
Both the nucleus (mitosis) and the cytoplasm (cytokinesis) must be divided during
cell division in eukaryotes
Stages of Mitosis:
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Division of the nucleus or mitosis occurs first
Mitosis is an asexual method of reproduction
Mitosis consists of 4 stages --- Prophase, Metaphase, anaphase, & Telophase
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Prophase:
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Metaphase:
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Chromosomes line up in center or equator of the cell attached to kinetochore
fibers of the spindle
Anaphase:
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Chromosomes become visible when they condense into sister chromatids
Sister chromatids attach to each other by the centromere
Centrioles in animal cells move to opposite ends of cell
Spindle forms from centriole (animals) or microtubules (plants)
Kinetochore fibers of spindle attach to centromere
Polar fibers of spindle extend across cell from pole to pole
Nuclear membrane dissolves
Nucleolus disintegrates
Kinetochore fibers attached to the centromere pull the sister chromatids apart
Chromosomes move toward opposite ends of cell
Telophase:
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Nuclear membrane forms at each end of the cell around the chromosomes
Nucleolus reform
Chromosomes become less tightly coiled & appear as chromatin again
Cytokinesis begins
Cytokinesis:
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Cytoplasm of the cell and its organelles separate into 2 new daughter cells
In animals, a groove called the cleavage furrow forms pinching the parent cell in two
In plants, a cell plate forms down the middle of the cell where the new cell wall will
be
Summary of Mitosis:
Interphase
Early Prophase
1. Cell matures & carries on
1. Chromosomes condense &
normal activities
2. DNA copied & appears as
chromatin
3. Nucleolus visible
become visible
2. Centrioles separate &
spindle starts forming
Late Prophase
Metaphase
1. Spindle forms with aster at each
pole
2. Nuclear membrane &
nucleolusdisintegrate
3. Centromere of chromosomes
attaches to spindle fibers
1. Chromosomes line up at the
equator of the cell attached
to kinetochore fibers of
spindle
Telophase/Cytokinesis
Anaphase
1. Centromeres split apart
2. Homologs move to opposite poles
of the cell
1. Nuclear membrane &
nucleolus reform
2. Cell pinches into 2 cells in
animals
3. In plants, a cell plate
separates the 2 new cells
Cancer is Uncontrolled Mitosis:
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Mitosis must be controlled, otherwise growth will occur without limit (cancer)
Control is by special proteins produced by oncogenes
Mutations in control proteins can cause cancer
Meiosis & Sexual Reproduction
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Reduces the number of chromosomes in new cells to half the number in the original
cell
New cells have a single copy of chromosomes (23 total) but are not identical to each
other or the original parent cell
Used for making gametes ( sperm and eggs) with the haploid or n number
In meiosis, cells divide twice after a single DNA duplication
Meiosis I separates homologs & the Meiosis II separates sister chromatids
Meiosis I stages are Prophase I, Metaphase I, Anaphase I, & Telophase I
Meiosis II stages are Prophase II, Metaphase II, Anaphase II, & Telophase II
Produces 4 haploid cells or gametes
When a sperm fertilizes an egg to form a zygote, the diploid number of chromosomes
is restored (23 + 23 = 46)
Egg cells or ova (ovum, singular) are larger , nonmotile cells
Gametoogenesis is meiosis producing eggs & occurs in the female's ovaries
Oogenesis
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Sperms contain less cytoplasm so they're smaller & have a flagellum to swim to the
egg
Spermatogenesis is meiosis producing sperm cells & occurs in the testes
Spermatogenesis
Meiosis I:
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The cell that undergoes Meiosis I is a primary spermatocyte or oocyte
Prophase I:
o Chromosomes coil tightly & are visible
o Nuclear membrane & nucleolus disintegrate
o Spindle forms
o Synapsis (joining) of homologous chromosomes occurs making tetrads
o Kinetochore fiber forms on each chromosome
o Chromosomes in tetrad exchange fragments by a process called crossing over
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Metaphase I:
o Tetrads become aligned in the center of the cell attached to spindle fibers
Anaphase I:
o Homologous chromosomes separate
Telophase I:
o May not occur in all species
o Cytokinesis occurs producing 2 cells
o In females, 2nd cell in females is called the 1st Polar Body
o 1st Polar Body dies due to uneven splitting of the cytoplasm
Prophase II:
o Cells called Secondary Spermatocytes or oocytes
o DNA is not copied before cell divides
o Chromatids attach to spindle fiber
Metaphase II:
o Chromosomes become aligned in the center of the cell attached to spindle
fibers
Anaphase II:
o Sister chromatids separate randomly
o Called independent assortment
Telophase I:
o Cytokinesis occurs producing 4 cells in males called spermatids
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Spermatids mature & form flagellum to become sperm
Cytokinesis in females produces a 2nd Polar Body that dies and an ootid
Ootids mature to become ovum or egg
Asexual & Sexual reproduction:
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Evolution is the slow process of change in living populations over time
Variations are differences that occur due to crossing-over among members of
a sexually reproducing population
Variations are important to the survival of individuals in a population (some must
survive to reproduce)
Asexually reproducing organisms rarely show variations because the organisms have
identical genes