Download Characteristic #4

Characteristic #4
All living things reproduce using the same genetic mechanism (code)
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Each organism reproduces its own species
Offspring inherit genetic instructions from their parents through a molecule called DNA
Offspring can differ from their parents (genetic variation)
Mitosis & Meiosis
Mitosis and meiosis are processes of making new cells from old cells
Both involve passing on genetic information from the old to the new cells
Genetics is the study of the transfer of information at the level of the cell.
Genetic information is found on DNA molecules within each cell nucleus
DNA is divided into chromosomes
Chromosomes are found in the nucleus of the cell
Each organism has a different number of
chromosome pairs (from one pair to hundreds of
pairs)
Humans have 23 pairs of chromosomes (see right)
Chromosomes come in homologous (matching)
pairs
Homologous chromosomes are two chromosomes
that have the same genes
Chromosomes are divided into genes
Genes control traits
New Terminology
Diploid: a body cell that has homologous pairs of chromosomes. Human diploid cells
have 23 chromosome pairs (46 chromosomes). The picture above shows the
chromosomes in a diploid cell.
Autosome: body cells (every type of cell except for sex cells). All autosomes are diploid.
Human autosomes have 46 chromosomes.
Haploid: a sex cell that has only one set of chromosomes (one from each pair). Human
haploid cells have 23 chromosomes.
Gamete: sex cell (sperm, egg) that are haploid
Mitosis & the Cell Cycle
The cell cycle and mitosis are required for growth, repair, and replacement of cells.
 DNA passes from old to new cells during the cell cycle
 Before mitosis can begin, each parent cell must duplicates
its DNA by making two copies of each chromosome
 After the DNA in the diploid cell is copied, the cell splits in
half, with one copy of DNA in each of the two new diploid
daughter cells
 Each new daughter cell is identical to the parent cell from
which it was created
 Mitosis creates autosomes (body cells)
How Does the Cell Cycle Work?
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Prior to division, the DNA relaxes in the nucleus. This is the Interphase stage
Before dividing, each chromosome creates a copy of itself that remains attached to the
original chromosome
The copies thicken to form in an “X” shape
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After interphase, the nucleus of the cell divides in a series of four steps called mitosis
o Prophase: Nucleus dissolves, and fibers attached to centrioles stretch across the
cell
o Metaphase: The chromosomes line up on the cell equator, and the fibers attach
to them from both sides
o Anaphase: The fibers shorten, splitting the chromosomes in half and pulling one
half to each pole
o Telophase: A new nucleus begins to form around each set of chromosomes, and
the cell starts to split in half
After mitosis, the cell splits into two daughter cells in a step called Cytokinesis
o In animal cells, a cleavage furrow develops and the cell simply splits in two
o In plants, a cell plate grows across the center of the cell, dividing it in half
Summary
o Mitosis is a way for cells to pass information to new autosomal cells within the
same organism
o The cell cycle & mitosis result in two diploid daughter cells that are identical to
the original diploid parent cell
o Cell cycle and Mitosis are important in growth, replacement, and repair of all
body cells
Meiosis
Meiosis is the creation of gametes (sperm and eggs) to be used in sexual reproduction
When the sperm fertilizes the egg, mom’s & dad’s DNA is combined together to produce
a zygote (one-celled offspring) that will grow into a fetus, and then a baby
To produce a baby with a normal
# of chromosomes, each sperm
and egg must have half the
number of chromosomes
Gametes (sex cells): Sperm and
eggs that are found only in the
testicles & ovaries. Gametes are
haploid - they have 23
chromosomes (no pairs) and are formed through meiosis
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Differences between gametes and autosomes :
o Gametes are sex cells - Autosomes are body cells
o Gametes are only in the testicles and ovaries - Autosomes are found
everywhere else in the body
o Gametes are haploid - Autosomes are diploid
o Gametes are made from meiosis - Autosomes are made from mitosis
o Meiosis produces 4 different haploid gametes - Mitosis produces 2 identical
diploid body cells
How does Meiosis Work?
The cell makes an extra copy of DNA as it does in the cell cycle
Chromosomes “cross over” to exchange
information before the first division. In
crossing over, the homologous
chromosomes get so close together that
pieces of each chromosome actually swaps
from one chromosome to the other.
The cell divides two times.
o During the first division, one of each
homologous pair of chromosomes
goes into each of the two cells (the
pairs are separated from one
another).
o In the second division, the two copies of each chromosome are divided and end
up in different cells
At the end of meiosis, there are 4 haploid daughter cells
The two chromosomes from each pair will end up in different daughter cells
After meiosis, each daughter cell will have half as many chromosomes as the parent.
After mitosis, the daughter cell will have the same number of chromosomes as the parent.
Meiosis vs. Mitosis
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If the parent cell has 60
chromosomes, the 4
daughter cells will have
30 chromosomes each
after meiosis
If the parent cell has 60
chromosomes, the 2
daughter cells will also
have 60 chromosomes
each after meiosis
Crossing Over
Homologous
chromosomes
move very close
together… so close
that they
sometimes they
swap pieces.
This creates brand
new combinations
of genes on each
chromosome
Crossing over leads to genetic variation – the offspring will have different traits than either
of their parents
Other Reproductive Issues
Grab Bag of Knowledge
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Selective Breeding: selective breeding is when humans breed specific individuals with
specific mates to exaggerate or produce certain traits. For example, dairy cows have
been modified through selective breeding for large udders and high milk production. As a
result, many cows have udders so large that they cannot walk without swinging their legs
out to the side.
Asexual reproduction is when one
parent produces one or more
identical offspring. No gametes or
sex is involved, and there is no
variation of traits. Examples of
Asexual Reproduction: (1) A branch of
a tree grows into a new tree when
planted; (2) Hydra create offspring
that grow from their body and then
pinch off
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Sexual reproduction involves the
fertilization of an egg by a sperm
(two parents). Offspring are different
from the parents and show genetic
variation.
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Animal Life Cycles: some organisms such as plants and animals go through changes in
form as they grow. Three types of life cycles are incomplete metamorphosis, complete
metamorphosis, and alternation of generations.
o Incomplete metamorphosis has 3 stages:
egg, nymph, adult. Grasshoppers use
incomplete metamorphosis.
o Complete metamorphosis has 4 stages: egg,
larvae (caterpillar), pupae (cocoon), adult.
Complete metamorphosis is the life cycle that
butterflies go through.
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Alternation of Generations:
Plants use alternation of
generations because they
alternate between a diploid and
haploid life phase. This is the
alternation of generations.