Download Sexual Reproduction & Meiosis

Sexual Reproduction &
Meiosis
SBI3U
Mr. Kleiman
What is Sexual Reproduction?
• Sexual Reproduction
– Occurs when two different parents reproduce
to form unique offspring
(different from their parents and siblings).
Mr. Kleiman
What determines the
TRAITS of the offspring?
• DNA codes for all traits
• The code for each trait is called a GENE
• Offspring receive a randomized
combination of genes from their parents
• Some genes will be expressed, while
others will not
• All genes that are passed on from one
generation to another are called
HEREDITARY
What determines the
TRAITS of the offspring?
• Even traits that are not expressed in either
parent, but are coded for in their DNA, are
hereditary
blond??
Which traits do you get?
• Before we examine how and why some
traits are expressed while others are not,
we must first understand how traits are
passed on from one generation to another
• Let’s review sexual reproduction and the
process of meiosis
Gametes
sperm
egg
Woo
hoo!
Zygote
Sexual Reproduction
• Gametes join to form a Zygote
egg
DNA
sperm
DNA
Gametes
develops into
body cells
Combined
DNA
Zygote
What does DNA do?
• DNA is a nucleic acid
containing 4 different
nitrogenous bases (A, T, G, C)
• The sequence of bases is called the
GENETIC CODE!
– this code tells your cells what to do
and contributes to all of your physical
traits
• Eg. hair/eye colour, shoe size, determines
if you are male or female
What does DNA do?
• Scientists also believe that your DNA has an
impact on how you think and how you behave.
The code
Base
G
P S
A
T
C
P S
P S
P S
Sugar
Phosphate
Nucleotide
P
P S P S P S P S P S P S P S P S P S
Genetic Code
C
T
A
G
C
C
G
C
T
T
A
G
G
base
G
S
G
A
C
C
P S P S P S P S P S P S P S P S P S
Template Strand
Naming the DNA
Chromosome
Chromatin
DNA with associated proteins
as it is found in the nucleus.
Naming the DNA
Chromosome
One particular bundle of DNA
(every species has it’s own number of chromosomes)
Naming the DNA
Chromosome
CanOne
be coiled
up during
cell
particular
bundle
of division.
DNA
(easier
to move
around
without
getting
tangled up)
(every
species
has
it’s own
number
of chromosomes)
Naming the DNA
Chromosome
copy during
copied
cell division
Naming the DNA
Sister Chromatids
copy
Identical chromosomes bound together by a
centromere
Sorting out the DNA
• Chromosomes are bundles of DNA
• Every species has a different number of
chromosomes
How many chromosomes do humans have?
46
Chromosomes
Sorting out the DNA
• If you got 46 chromosomes from your mother
and you got 46 chromosomes from your father
you would have…
92
Chromosomes!!
Sorting out the DNA
• If you then reproduced, your offspring would get
92 chromosomes from you and
92 chromosomes from your partner… that’s…
184
Chromosomes!!
See the
problem?
Sorting out the DNA
Question:
If we combine the chromosomes from each parent
during sexual reproduction, why doesn’t the
chromosome number double?
Answer:
Gametes must
have half the
number of
chromosomes as
body cells
example (not human)
egg
10
10
20
sperm
Sorting out the DNA
• Try these
– How many chromosomes in each gamete?
Gametes
Zygote
23
23
39
39
12
12
46
78
24
Human
Chicken
Rice
This means that EACH cell in your body has one copy of DNA
from your mother and one copy of DNA from your father…
BUT ONLY HALF OF THEIR GENES!!!!
How?
• How do gametes get half the number of
chromosomes?
By the process of
Meiosis
Homologous Chromosomes
Gamete from mother
Gamete from father
Homologous Chromosomes
Homologous chromosomes are NOT identical,
Zygote
but code for
the same traits
Homologous
chromosomes
Meiosis I
Prophase I
Meiosis I
Metaphase I
Line up
homologous
chromosomes
Completely unique
DNA created
Crossing Over
Meiosis I
Metaphase I
Meiosis I
Anaphase I
Meiosis I
Telophase I
Meiosis II
Prophase II
DNA is not copied
Meiosis II
Metaphase II
Meiosis II
Anaphase II
Meiosis II
Telophase II
Meiosis… the end result
4 unique cells
created from one
body cell
Each cell has half
the normal number
of chromosomes
(gametes)
Key Features of Meiosis
• Meiosis I
– Copy chromosomes
– Line up and separate homologous
chromosomes
– Crossing over between homologous
chromosomes
• Meiosis II
– Separate sister chromatids
(normal cell division)
Why are you random?
There are BUILT IN MECHANISMS in meiosis to
create variation:
• Crossing over events which generate an
infinite number of new random chromosomes
• Random assortment of your parents
homologous chromosomes in their gametes
during meiosis I and sister chromatids during
meiosis II
Why are you random?
Your specific combination of genes is a result of…
• One of many sexual encounters on a
particular day at a particular time
• One of millions of sperm which meet with
one of thousands of eggs
Why are you random?
DNA doesn’t define you…
• Your environment, choices and random
chance all play a large role in your
– Gene expression
– Physical Development
– Experiences
– Health
– Abilities
Identical?
Errors in Meiosis
Nondisjunction –
Down Syndrome
Down Syndrome
Down Syndrome
Nondisjunction –
Turner Syndrome
Nondisjunction –
Turner Syndrome