Chromosomes and Cell
Reproduction
Chapter 6
Dr. Altstiel
Naples High School
Bell Ringer
• Write a single sentence that reflects the
understanding of each of the following
terms.
– Haploid
– Diploid
– Zygote
Bell Ringer Answer
During fertilization, haploid
gametes combine to form a
diploid zygote.
Objectives
• IDENTIFY four examples of cell division in
eukaryotes and one example in prokaryotes.
• DIFFERENTIATE between a gene, a DNA
molecule, a chromosome, and a chromatid.
• DIFFERENTIATE between homologous
chromosomes, autosomes, and sex chromosomes.
• COMPARE haploid and diploid cells.
• PREDICT how changes in chromosome number
or structure can affect development.
Chromosomes: Formation of New
Cells by Cell Division.
• About 2 trillion cells reproduced in the
human body daily.
• New cells formed by older cells dividing
• About 25 million cells new per second
• DNA must be present in each new cell
• DNA must be copied before the cell divides
Prokaryotic Cell Reproduction
•
Binary Fission – form of asexual
reproduction.
–
–
–
–
Bacteria cell division
Asexual
Identical offspring
Occurs in two stages:
1. DNA copied
2. Cell splits
Gene
• Gene – a segment of DNA that codes for a
protein or RNA molecule.
• Single molecule of DNA has thousands of
genes.
• Play an important role in determining how a
person’s body develops and functions.
Eukaryotic Cell Reproduction
• Strands of DNA appear as
spaghetti called chromatin.
• When cells are ready to divide,
material composed of DNA and
protein, is tightly compacted
forming a structure called a
chromosome.
• Two exact copies of DNA that
make up each chromosome are
called chromatids.
– Two chromatids attached at point
called a centromere.
Number and structure of
Chromosomes affect development
• Humans have 2 copies of 23 different
chromosomes (46 total)
– Homologous chromosomes are similar in size,
shape and genes
– Each one in a set comes from mom; other from
dad
Chromosome Number
• Karyotype – a set of chromosomes.
Humans have 23 pairs of chromosomes =
46.
– Haploid – cells that contain only one copy of
each chromosome.
• Haploid Number = n
• Humans, n = 23
• Zygote – fusion of two haploid gametes by a
process called fertilization – a diploid zygote is a
fertilized egg.
– Diploid – when cells in the body contain 23
pairs of chromosomes. Practically all cells in
the body are diploid.
• Diploid Number = 2n
• Humans, 2n = 46
• Diploid cells (somatic body cells) contains
two sets of chromosomes
• Haploid cells (sex cells) contains one set of
chromosomes
• Two haploid sex cells come together during
fertilization and make a diploid zygote
• 22 chromosomes are called Autosomes and
one of the 23 chromosomes is called a sex
chromosome.
Sex Chromosome
• Determines whether you are male or female.
Sex chromosomes designated as either X or
Y.
– Female = XX
• Can only produce eggs that contain an X
chromosome.
– Male = XY
• Can produce sperm with either an X or Y
chromosome. Therefore, male’s sperm is sex
determiner.
• Autosomes – chromosomes not directly
involved in determining the sex of an
individual (22 pairs).
Determining your sex
•
•
•
•
Autosomes DO NOT determine your sex
Sex chromosomes determine your sex
XX = female
XY = male
Change in chromosome number
• Abnormalities in chromosome number leads
to genetic disorders such as Down
Syndrome
• 1 in 1500 births of mothers younger than 30
• 1 in 750 births of mothers 30-45
• 1 in 16 over 45
Homework
• Section Review 1-5, Page 124.
Cell Cycle
• The cell cycle is an
ordered set of events,
culminating in cell growth
and division into two
daughter cells. It is a
repeating sequence of
cellular growth and
division during the life of
the organism.
• A cell spends 90% of its
time in the first three
phases of the cycle.
• Non-dividing cells not
considered to be in the cell
cycle.
Cell Cycle Cont.
• The stages, pictured to the
left, are G1-S-G2-M.
– G1 stage = "GAP 1".
Occupies the major portion
of the cell’s life.
– S stage = "Synthesis". This
is the stage when DNA
replication occurs.
– G2 stage = "GAP 2".
Growth phase. Preparations
made for nucleus to divide.
– M stage = "mitosis", nuclear
(chromosomes separate) and
cytoplasmic (cytokinesis)
division occur.
•Interphase occurs in the stages G1, S, and G2. No
divison. Only growth and development.
•Cytokinesis – process in cell division in which the
cytoplasm divides.
Controlling the Cell Cycle
• Cancer arises from faulty
cell cycle; when control is
lost.
– Tumor – cell that grows
and divides without
restraint.
• Benign – harmless, remains
in region originally formed.
• Malignant – life threatening,
cancerous.
– Cancer – uncontrolled cell
division and growth.
– Caused by defects in the
genes regulating cell
division called mutation.
Mitosis and Cell Division
• Cells grow and
develop. When they
reach a certain size, it
will either stop
growing or divide.
• Cells that divide must
first make a copy of its
genetic information
first.
Mitosis
• The process by which
the nucleus of a
eukaryotic cell divides
to form two new
nuclei. Four Phases:
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
• Cytokinesis – cell
division. Cell divides
into two cells, each
with its own nucleus.
• Interphase
Mitosis Phases
– The cell is engaged in
metabolic activity and
performing its
prepare for mitosis.
– Chromosomes are not
clearly discerned in
the nucleus, although
a dark spot called the
nucleolus may be
visible.
– The cell may contain
a pair of centrioles
(or microtubule
organizing centers in
plants).
Mitosis Phases Cont.
• Prophase
– Chromatin in the nucleus
begins to condense and
becomes visible in the
light microscope as
chromosomes.
– The nucleolus disappears.
– Centrioles begin moving
to opposite ends of the cell
and fibers extend from the
centromeres.
– Some fibers cross the cell
to form the mitotic
spindle.
Mitosis Phases Cont.
• Metaphase
– Spindle fibers align
the chromosomes
along the middle of
the cell nucleus. This
line is referred to as
the metaphase plate.
– This organization
helps to ensure that in
the next phase, when
the chromosomes are
separated, each new
nucleus will receive
one copy of each
chromosome.
Mitosis Phases Cont.
• Anaphase
– The paired
chromosomes separate
at the kinetochores and
move to opposite sides of
the cell.
Mitosis Phases Cont.
• Telophase
– Chromatids arrive at opposite
poles of cell, and new
membranes form around the
daughter nuclei.
– The chromosomes disperse
and are no longer visible
under the light microscope.
– The spindle fibers disperse,
and cytokinesis or the
partitioning of the cell may
also begin during this stage.
Mitosis Phases Cont.
• Cytokinesis
– In animal cells,
cytokinesis results when a
fiber ring composed of a
protein called actin
around the center of the
cell contracts pinching the
cell into two daughter
cells, each with one
nucleus.
– In plant cells, the rigid
wall requires that a cell
plate be synthesized
between the two daughter
cells.
Class Work
•Section Review 1-4,
pg 132.
Meiosis
• Meiosis is a two-stage form of nuclear
division in which the chromosome number
is halved.
– Meiosis I – reduces the chromosome number by
half.
• Chromosomes in a diploid cell resegregate,
producing four haploid daughter cells. It is this
step in Meiosis that generates genetic diversity.
– Meiosis II – separates identical copies of
chromosomes.
Meiosis I
• Prophase I
– DNA replication precedes
the start of meiosis I.
– Homologous
chromosomes pair and
form synapses, a step
unique to meiosis.
– The paired chromosomes
are called bivalents
– A bivalent has two
chromosomes and four
chromatids, with one
chromosome coming from
each parent.
Meiosis I Continued
• Metaphase I
– Bivalents align at the
metaphase plate.
– The orientation is
random, with either
parental homologue on a
side. This means that
there is a 50-50 chance for
the daughter cells to get
either the mother's or
father's homologue for
each chromosome.
Meiosis I Continued
• Anaphase I
– Chromosomes, each
with two chromatids,
move to separate
poles.
– Each of the daughter
cells is now haploid
(23 chromosomes),
but each chromosome
has two chromatids.
Meiosis I Continued
• Telophase I
– Nuclear envelopes
may reform, or the
cell may quickly start
meiosis 2.
Meiosis I Continued
• Cytokinesis
– Analogous to mitosis
where two complete
daughter cells form.
Meiosis II
Meiosis II Continued
Meiosis II Continued
Meiosis II Continued
Meiosis II Continued
Phases of Meiosis
Crossing-Over
DNA is exchanged between homologous chromosomes, resulting
in genetic recombination.
Class Work
• Section Review 1-4,
112.