Transcript Cell Reproduction Chapter 8

Cell Reproduction
Chapter 8
Topics:
Chromosomes
Cell Division
Meiosis






DNA: deoxyribonucleic acid
Found in nucleus of eukaryotes, DNA loop in
prokaryotes
Codes for proteins, cell functions
DNA + proteins = chromosome
Chromosomes composed of genes; gene is a
specific segment of DNA coding for one
particular protein
The protein(s) coded for determines an
organism’s characteristics

DNA structure is a coiled double helix; has
associated proteins
– Histones – proteins that help maintain the coiled
structure
– Non-histones – proteins that control DNA activities
In a non-dividing cell, DNA is diffuse & difficult
to see; called chromatin
 In a dividing cell, DNA becomes condensed into
chromosomes, much easier to see; called a

chromosome
DNA
DNA
Double helix
showing base
pairs 
DNA with associated proteins 
Chromosome

Chromosome – tightly coiled DNA +
proteins, seen during cell division
 Consist of 2 identical halves called
chromatids

Chromotids are connected by a

Genes located on chromosome
centromere
– What’s a gene again?
Chromosome Number
The number of chromosomes vary from
species to species; no correlation between
chromosome number and species
sophistication
 Some examples: (page 145 text)

– Human 46 Cat 32
Dog 78
– Lettuce 18 Gorilla 48 Fern 1,262

Animal chromosomes divided into
autosomes or sex chromosomes
Sex Chromosomes
Chromosomes that determine sex of
individual
 These chromosomes also carry genes
determining other characteristics
 Humans: female XX; male XY
 Humans: have 2 sex chromosomes

Autosomes
All other chromosomes
 Carry genes determining characteristics of
individual
 Humans have 44 autosomes

Karyotype
“picture of chromosomes”

Female
Male
Chromosomes

Focus on human
– Has 46 chromosomes: 2 sex chromosomes & 44 autosomes or
23 pairs of chromosomes: 1 pair sex chromosomes & 22 pairs of
autosomes
– Each individual receives 23 chromosomes from female parent &
23 chromosomes from male parent
– The 23 chromosomes from mom match the 23 chromosomes
from dad; called homologues or homologous pairs
– The genes on each chromosome code for the same traits

Each homologous pair contain gene sequences coding
for the same traits
– Look at karyotypes of normal individuals on prior slide

Karyotype – term for pictomicrograph of chromosomes
Diploid v. Haploid

Diploid – description of cells having 2 sets of
chromosomes
– Human: all body cells are diploid
– Abbreviated as “2n”
– Human diploid number is 46

Haploid – description of cells having 1 set of
chromosomes
– Humans: only sex cells are haploid (egg, sperm)
– Abbreviated as “1n”
– Human haploid number is 23
Prokaryotes
Prokaryotic chromosomes consist of a
circular DNA molecule with associated
proteins
 Most prokaryotes have only 1
chromosome
 Chromosome DNA loop attached to cell
membrane
 Prokaryotic DNA is haploid

Prokaryotic Cell Division
Called Binary Fission
 Steps in process

– DNA loop copies itself; so cell has 2 copies
– Cell continues to grow, making copies of cellular
contents
– When cell reaches approx 2X original size, a cell wall
forms between 2 copies of DNA
– As wall forms, it divides cell into 2 equal halves
– Called daughter cells, each identical to other
Eukaryotic Cell Division

Two types of cell division in eukaryotes

Mitosis
– Results in production of daughter cells that
are identical to parent cell
– Results in 2 diploid daughter cells

Meiosis
– Results in production of daughter cells that
are not identical to parent cell
– Results in 4 haploid daughter cells
Mitosis
Occurs in body cells (somites) for growth &
repair
 Occurs in unicellular organisms for reproduction
 Yields 2 diploid daughter cells that are
genetically identical to parent cell



Mitosis is only a small part of the life cycle of a
cell
Cell cycle is the repeating events that make up
the life cycle of a cell
Cell Cycle

Interphase (time between cell divisions)
– G1 phase (cell growth)
– S phase (DNA copied)
– G2 phase (cell growth; prep for cell division)

M phase (division of cell nucleus)
–
–
–
–

Prophase
Metaphase
Anaphase
Telophase
Cytokinesis (division of cell cytoplasm)
Cell Cycle
Interphase

G1 phase – “G” stands for “gap”, referring
to the gap between cell divisions
– Cell grows, organelles copied, necessary
proteins manufactured/stored
S phase – DNA copied during this phase; if
cell is to divide then each cell needs
complete set of DNA
 G2 phase – any further cell growth occurs
here, cell prepares for division

Interphase
Cells in interphase
 Onion cells & whitefish cells

Prophase


First stage of cell division
Occurrences:
– DNA coils into typical chromosome shapes, can be seen using
light microscope
– Remember chromosome composed of 2 chromatids connected
by centromere (but the DNA amount is doubled!)
– Nuclear membrane & nucleolus break down
– 2 centrosomes (containing 2 each centrioles) form [Exception:
plant cells have no centrioles]
– Spindle fibers radiate between centrosomes attaching to
chromosomes and other cell organelles
– Centrosomes migrate to poles (opposite sides of cell) with the
mitotic spindle arrayed between them
Prophase

Cells in prophase
Interphase & Prophase
Metaphase
Second stage of cell division
 Occurrences:

– Chromosomes migrate to equator of cell
– Kinetochore – disk shaped protein found in the center
of each centromere (remember the centromere
connects the chromatids); kinetochore fibers connect
the centromere to one of the centrosomes
– Polar fibers connect centromeres of adjacent
chromatids along equator
– It is the kinetochore & polar fibers that help align the
chromosomes along the cell’s equator
Metaphase

Cells in metaphase
Anaphase
Third step in cell division
 Occurrences:

– The centromere of each chromatid separates
– One chromatid pulled to one pole; other chromatid of
that pair pulled to opposite pole
– Chromatid considered a complete chromosome now
– Remember the DNA amount was doubled in the S
phase of Interphase, so the “chromatid” that
migrated to the pole has 2n DNA amount
– The mitotic spindle ensures the proper migration of
DNA to poles
Anaphase

Cells in anaphase
Telophase
Fourth step in cell division
 Occurrences:

– Spindle fibers disassemble
– Chromosomes become diffuse (now called
chromatin)
– Nuclear membrane reforms
– Nucleolus reforms
Telophase

Cells in telophase
Telophase
Cytokinesis
Step where the cell physically divides
 Animal cell

– Cleavage furrow forms where cell membrane begins
to pinch together
– Cell components equally divided to both daughter
cells
– Each daughter cell has own nucleus that formed
during telophase

Plant cell
– Cell plate forms to separate cell contents
– Each cell has own nucleus formed in telophase
Cytokinesis

Cells undergoing cytokinesis
Meiosis
Occurs in germ cells of the reproductive organs,
testes or ovaries
 Produces 4 haploid gametes (sperm, ova)
 Unlike mitosis where there is one division phase,
meiosis has 2 divisions

– Meiosis I
– Meiosis II

Cells undergoing meiosis prepare by going
through interphase (same events occur here,
mainly duplication of chromosomes)
Meiosis I

Prophase I
– Cell enters with duplicated chromosomes
– Events of prophase I are same as prophase of mitosis except
that synapsis occurs
– Synapsis – lining up of homologues; so the chromatids of
homologous chromosomes are lined up next to each other;
these paired homologues called tetrads
– Important because this aligns corresponding genes on the
chromatids, the chromatids touch and genetic material is
exchanged
– Crossing-over is the term for this exchange of genetic material
between homologous chromosomes
– This results in genetic recombination
Meiosis I, con’t

Metaphase I
– Tetrads line up randomly along equator of cell, just as the chromatids
did in mitosis metaphase
– Spindle fibers form, extend from centrioles to centromeres of each
tetrad

Anaphase I
– Tetrads (homologous pairs) pulled apart; one to one pole, one to the
other
– Because they are randomly aligned along equator, the homologue
pulled to a pole is random; called Independent Assortment

Telophase I
– Homologues reach their respective pole
– Cell begins to pinch together; cleavage furrow forms

Cytokinesis
– Cell splits into two, both cells are haploid (1n)
– Because each cell received only one of each homologous pair
Meiosis I
Remember this as meiosis
continues…
When cytokinesis occurs in meiosis I, 2
haploid cells formed, each having only one
of the homologous chromosomes (but
remember that the chromosome carries
with it a copy)
 The nuclear membrane does NOT reform
and cells does NOT begin another
interphase
 Cells immediately go into prophase II

Meiosis II

Prophase II
– Spindle fibers form and move chromosomes to equator of each
cell

Metaphase II
– Chromosomes aligned along equator of each cell

Anaphase II
– Chromatids now separate and are pulled to one of the poles

Telophase II
– Nuclear membrane reforms

Cytokinesis
– Cleavage furrow forms and each cell splits into two
– Yield 4 haploid cells
Meiosis II
Gametogenesis

In females, meiosis occurs in ovaries
– Process called oogenesis
– Yields 4 haploid cells: 1 egg and 3 polar
bodies

In males, meiosis occurs in testes
– Process called spermatogenesis
– Yields 4 haploid cells: 4 spermatids that
mature into 4 sperm cells
Spermatogenesis
Begins with the onset of puberty and release of
male sex hormone, testosterone
 Continues throughout the lifetime of the
individual
 It takes only a few weeks for each gamete to
reach maturity and become a sperm

– So, every sperm in a male is only a few weeks old
Occurs in the testes
 Each mature sperm is haploid, 1n

Oogenesis






Begins during gestation in the fetal ovaries, but stops at
Prophase I; cells are primary oocytes
So female infant is born with ovaries containing primary
oocytes that are in “arrested” development awaiting
“activation”
Meiosis does not restart until puberty about 10 – 14
years later
With each 28 day cycle (mensus), only one primary
oocyte matures to become 1 egg (1n) and 3 polar bodies
The egg receives all the cellular contents making it quite
large; the polar bodies are “discarded” cells which die
and are resorbed by the body
Note: females are born with a finite number of primary
oocytes which formed during gestation; this makes the
egg as old as the woman; older women more likely to
produce ova with mutations
Fertilization




Fertilization – union of egg and sperm
Occurs in the fallopian tube of female
Fertilized egg – zygote; is diploid or 2n
Zygote implants into uterine lining; which
became engorged with blood during the 28 day
cycle
– If zygote implants, the ovaries & other endocrine
glands produce hormones to maintain the pregnancy
– In human, gestation is ~9 months

No zygote, uterine lining sloughed off as
menstrual flow