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Meiosis - Chapter 10.1 and 11.3
& Chromosomes
Chromosome Structure
•DNA coils around histones (proteins) to
form nucleosomes,
•This coils to form chromatin fibers.
•The chromatin fibers supercoil to form
chromosomes that are visible in the
metaphase stage of mitosis.
https://www.youtube.com/watch?v=tsVHWbXqum8&t=249s
How many chromosomes do
we have in our cells?
▪ Human body cells have 46 chromosomes
▪ Each parent contributes 23 chromosomes
▪ Homologous chromosomes - a pair of
chromosomes that have the same genes
Homologous Chromosomes
-Same length
•-Same centromere
position (middle)
•-One maternal and one
paternal chromosome
Homologous Chromosomes
▪ Carry the same genes that
control the same inherited
traits
▪ Alleles are the variations of
one gene
–Ex: eye colors can brown,
blue, green, hazel, etc….
▪ You inherit one chromosome
from each parent!
DO NOW
Label the following structures:
Pairs #1-23
23 pairs =
46 total
chromosomes
These are your
chromosomes!
1 chromosome in
each pair comes
from your mother
and father!
Unduplicated homologous
chromosomes
Duplicated homologous
chromosomes
Karyotyping
• A micrograph that shows pairs of homologous
chromosomes in decreasing size
● 46 chromosomes, 23 pairs
○ Pairs #1 - 22 are autosomes
○ Pair #23 are the sex
chromosomes
■ XX or XY
Karyotype
Chromosomal Mutations nondisjunction
Monosomy – Results
in a sex cell with one
less chromosome
Trisomy – Results in a sex chromosome
with one extra chromosome
Do Now
•1. How many
autosomes?
•2. How many sex
chromosomes?
•3. What is the sex of
this patient?
•4. Monosomy,
Trisomy?
•5. Is this individual
normal?
Before we start genetics, let's
review the process of what makes
us all look different compared to our
siblings, family, and classmates!
Sexual Reproduction
Sexual reproduction creates genetic
variability. That is why we all look different
and you look different from your siblings!
Define the following words…
• Meiosis:
– Sexual reproduction-production of gametes
• Gamete:
– Sex Cells (egg = females & sperm = males)
– contain half the number of chromosomes (23)
• Somatic Cells:
– Body Cells
– Contain 46 chromsomes
• Haploid:
– Half the chromosome number (n)
– 1 of each pair of chromosomes (23)
– Gametes are haploid
• Diploid:
– 2 of each type of chromosome (2n)
– Somatic cells are diploid
• Fertilization:
– Sperm enters egg
• Zygote:
– Fertilized egg (diploid)
Think - Pair - Share
Can you apply terms learned on the past
few slides to this picture?
Meiosis
• The sexual life cycle in animals involves meiosis
• Meiosis produces
gametes (23
chromosomes).
• When gametes
combine in
fertilization, that
creates a zygote.
• A zygote has 46
chromosomes
Meiosis
• 2 Divisions
Meiosis I - separation of homologous chromosomes
Meiosis II - separation of sister chromatids
DNA Replication before Meiosis 1
Before Meiosis begins, all DNA must be replicated
Meiosis 1
1. Crossing over occurs
1. Homologous
chromosomes
separate, randomly!
46
Duplicated
23
duplicated
Crossing over = the exchange of genes between
homologous chromosomes, resulting in a mixture of
parental characteristics in offspring.
Crossing over
• The same genes “switch”, which allows for
unique gametes to form, different from the exact
chromosomes given from the mother and father.
• Occurs during Meiosis 1, Prophase 1
Meiosis 2
1. Sister chromatids
separate into 4
different gametes,
randomly.
Each gamete has a
unique combination
of genes!
23
duplicated
23 unduplicated chromosomes in each gamete
Meiosis 1 & 2 - Females (Eggs) and Males (Sperm)
What creates genetic variability?
1. Crossing over - switching of genes during
meiosis 1 creates a unique combination of genes
1. Independent Assortment - chromosomes
separate into gametes randomly during meiosis
1 and meiosis 2
1. Random Fertilization - random sperm and egg
combine to make a unique gamete