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Honors Biology
Unit 5
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}  Genome:
Complete complement of
an organism’s DNA.
◦  Includes genes (control traits) and noncoding DNA organized in
chromosomes.
Genome > Chromosome > DNA > Genes
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}  Heredity
– way of
transferring genetic
information to
offspring
}  Gene
– “unit of
heredity”.
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}  Fusion
of two gametes to
produce a single zygote.
}  With
exception of selffertilizing organisms (e.g. some
plants), zygote has gametes
from two different parents.
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} 
Total = 46 chromosomes (23 pairs)
44 autosomes + 2 sex chromosomes
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1. 
Autosomes:
◦  Code for most genes in your body (not sex
determining chromosomes)
◦  In humans chromosome #1-22
2. 
Sex Chromosomes:
◦  chromosomes that
determine what gender
you will be.
◦  #23 in humans:
–  Females are: XX
–  Males are XY
}  Gametes:
sperm/egg
◦ haploid (n)
◦  Haploid= Contains a single set of
chromosomes (23)
}  Zygote:
fertilized egg
}  Somatic
cell: any cell other than gametes–
◦  now diploid (2n)
◦  Diploid= Contains a two sets of chromosomes
(23x2=46)
All are diploid (2n)
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Meiosis KM
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Chromosomes exist in homologous pairs in
diploid cells.
}  homologous pairs: chromosomes that code
for the same genes but possibly have
different alleles (get one from each parent)
} 
Exception: Sex
chromosomes (X, Y).
All autosomes are in
homologous pairs.
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•  Now each
homologous
chromosome has a
sister chromatid
•  Still considered
diploid (2n) cell
because still just two
chromosomes (even
though they now
have a twin)
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All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of
copies of each
chromosome.
Diploidy
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}  Meiosis:
cell division process by
which the number of
chromosomes per cell is cut in
half
}  Why
does it occur: Meiosis is used
to produce the haploid(n) gametes
(sperm and eggs)
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}  Meiosis
reduces the number of
chromosomes by half.
}  Daughter
cells differ from parent,
and each other.
}  Meiosis
involves two divisions,
Mitosis only one.
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Meiosis 1: Separates homologous chromosomes
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•  produces recombinant chromosomes, mixing the
genes of the mother and father, recombining them.
•  Occurs during Prophase 1
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Meiosis II: Separates Sister Chromatids
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} 
} 
During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by:
◦  Shuffling of maternal and paternal
chromosomes and crossing over.
◦  No daughter cells formed during meiosis
are genetically identical to either mother
or father
◦  During sexual reproduction, fusion of the
unique haploid gametes produces truly
unique offspring.
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Number of combinations: 2n
e.g. if there are 2 chromosomes in haploid
2n = 4; n = 2
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2n = 22 = 4 possible combinations
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
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Male gamete formation Spermatogenesis
}  all
4 products of
meiosis develop into
sperm cells. They lose
most of their
cytoplasm, remodel
their cell shape, and
grow a long flagellum
(tail).
Female gamete formation - Oogenesis
}  In
females, most of
the cytoplasm goes
into 1 of the 4
meiotic products,
which becomes the
egg.
}  The
other 3 meiotic
cells are small “polar
bodies”, which
degenerate.
}  1.
More genetic diversity:
more potential for survival
of species when
environmental conditions
change.
◦ Shuffling of genes in meiosis
◦ Crossing-over in meiosis
◦ Fertilization: combines genes
from 2 separate individuals
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} 
2. DNA back-up and repair.
◦  Asexual organisms don't have back-up copies
of genes, sexual organisms have 2 sets of
chromosomes and one can act as a back-up if
the other is damaged. (some organisms have
more)
◦  Sexual mechanisms, especially
recombination, are used to repair damaged
DNA - the undamaged chromosome acts as a
template and eventually both chromosomes
end up with the correct gene.
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1. How does metaphase of mitosis differ from
metaphase I of meiosis?"
2. What happens as homologous chromosomes pair up
during prophase I of meiosis?"
3. What specific activities, involving DNA, occur during
interphase prior to both mitosis and meiosis? "
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4. Compare mitosis and meiosis on the following points:
"a. number of daughter cells produced.
"b. the amount of DNA in the daughter v. parent cell ""
"c. mechanism for introducing genetic variation."
5. What is a zygote and how is it formed?
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6. Summarize the process of meiosis in 4
sentences or less (include the following
in your summary)"
1.  What is the main purpose of Meiosis?"
2.  What does it start with and what does it end
with (include males and females)"
3.  Use and underline vocab words gamete,
diploid, haploid, crossing over, eggs, sperm,
homologous pairs, and sister chromatids"
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