Cell Reproduction 8.1
Reasons for Cell Division
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Cell division allows us to grow so we can pass on our genetic
information.
It allows us to repair & replace damaged cell parts
Large cells have a hard time moving nutrients across a cell
membrane so we need to limit cell size.
Chromosomes & Their Structure:
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DNA is organized into
chromosomes
made of protein & a long,
single, tightly-coiled DNA
molecule
visible only when the cell
divides
When a cell is not
dividing it is called
chromatin
DNA in eukaryotic cells
wraps tightly around
proteins called histones
to help pack the DNA
during cell division
Non-histone proteins help
control the activity of
specific DNA genes
Kinetochore proteins bind to centromere and attach chromosome
to the spindle in mitosis
Centromeres hold duplicated chromosomes together before they
are separated in mitosis
 Telomeres are the ends of chromosomes
which are important in cell aging
 each half of the chromosome is called a
sister chromatid
DNA of prokaryotes (bacteria) is one, circular
chromosome attached to the inside of the cell
membrane
Chromosome Numbers:
Human somatic cells (or body cells) have 23 pairs of chromosomes
or 46 chromosomes (diploid or 2n number)
The 2 chromatids of a chromosome pair are called homologues
(have genes for the same trait at the same
location)
Human reproductive cells or gametes (sperm &
eggs) have 23 chromosomes (haploid or n
number)
Every organism has a specific chromosome
number.
Organism
Human
Fruit fly
Lettuce
Goldfish
Chromosome
Number (2n)
46
8
14
94
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
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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
genetic locus is the position of a gene in a linkage map or on a
chromosome.
The Cell Cycle 8.2
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When a living organism needs new cells to
repair damage, grow, or just maintain its
condition, cells undergo mitosis.
Cells go through phases or a cell cycle during
their life before they divide to form new cells
Cell division includes mitosis (nuclear division)
and cytokinesis (division of the cytoplasm)
Stages of Mitosis
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Mitosis, also called karyokinesis, is division of the nucleus and
its chromosomes.
It is followed by division of the cytoplasm known as cytokinesis.
The Cell Cycle – involves both mitosis and cytokinesis
Interphase
A non-dividing phase that includes:
G1 stage  newly divided cells grow in
size,
S stage number of chromosomes is
doubled and appears as chromatin
G2 stage cell makes the enzymes &
other cellular materials needed for
mitosis
Cell division in Prokaryotes:
Ex: bacteria do not have a nucleus
Binary fission – divide cell into two identical new cells
Asexual method of reproduction
1. the chromosome, attached to cell membrane,
makes a copy of itself and the cell grows to about
twice its normal size
2. a cell wall forms between the chromosomes
3. the parent cell splits into 2 new identical daughter cells (clones)
Cell Division in Eukaryotes:
 Parent cell & resulting 2 daughter cells must have identical
chromosomes
 DNA is copied in the S phase of the cell cycle
 Organelles are copied in the Growth phases
 Both the nucleus and the cytoplasm must be divided during cell
division in eukaryotes
Stages of Mitosis:
P-M-A-T
Prophase:
 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
 Spindles form from centrioles (animals) or microtubules
(plants)
 Kinetochore fibers of spindle attach to centromere
 Polar fibers of spindle extend from pole to pole
 Nuclear membrane dissolves
 Nucleolus disintegrates
Metaphase:
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Chromosomes line up in center or
equator of the cell attached to
kinetochore fibers of the spindle
Anaphase:
 Kinetochore fibers attached to the centromere pull the
sister chromatids apart
 Chromosomes move toward opposite ends of cell
Telophase:
 Nuclear membrane forms at each end of the cell
around the chromosomes
 Nucleolus reforms
 Chromosomes become less tightly coiled & appear as
chromatin again
 Cytokinesis begins in late telophase
Cytokinesis:
 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
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In plants, a cell plate forms down the middle
of the cell where the new cell wall will be
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