Download Meiosis

Title:
Meiosis
How much do you think we have left of the course?
What dates are your exams?
Homework
Breadth in biology
Depth in biology
25th May
06th June
• To have brought your Science Summary Sheets up to date,
including past paper questions to match each area (record the
year, exam board, paper and question number for each).
• You then need to complete and self-assess at least 10 of these
past exam questions and bring in evidence of this on Tues 29th.
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Chromosome Structure
But you have 23 pairs of chromosomes, so...
Before
DNA replication
After DNA
replication
Gene for eye colour
Brown eyed allele
Blue eyed allele
Gene for enzyme A
Homologous chromosomes are the
same size and have the same genes, but
may have different alleles.
So what is the difference between a pair of homologous
chromosomes and a pair of sister chromatids?
Brown eyed
allele
Brown eyed
allele
Sister chromatids are joined at the centromere, are the same size,
have the same genes and have the same alleles.
Haploid Vs Diploid
• Diploid cells – Normal body cells – 2 homologous
chromosomes (pairs of chromosomes)
• Haploid cells – Sex cells - only one of each
homologous chromosome (singular chromosomes)
Haploid gametes
Diploid Zygote
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Chromosomes go
from single to
sister chromatids
Chromosomes go
from single to
sister chromatids
Division 1: homologous
chromosomes separate
Division 2: chromatids
separate
Single division – chromatids
separated
(12 chromatids)
(6 chromatids)
(3 chromatids)
Meiosis Animation
Whilst you are watching the video, write 3
questions to test each other with.
• What?
• When?
• Where?
• Why?
www.sumanasinc.com/webcontent/animations/
content/meiosis.html
Prophase I
Blocks of
genes are
swapped
Bivalents
A bivalent, sometimes referred to as a tetrad, is a pair of
associated homologous chromosomes held together by a
complex after chromosome replication.
© Pearson Education Ltd 2009
This document may have been altered from the original
Metaphase I
© Pearson Education Ltd 2009
This document may have been altered from the original
Anaphase I
© Pearson Education Ltd 2009
This document may have been altered from the original
Telophase I
© Pearson Education Ltd 2009
This document may have been altered from the original
Prophase II
© Pearson Education Ltd 2009
This document may have been altered from the original
Metaphase II
© Pearson Education Ltd 2009
This document may have been altered from the original
Anaphase II
© Pearson Education Ltd 2009
This document may have been altered from the original
Telophase II
Starting
diploid cell
© Pearson Education Ltd 2009
This document may have been altered from the original
Your Tasks
1. Cut out each of the cards showing the stages of
meiosis and put them into the correct order.
2. Complete the table on your revision sheet to
compare mitosis and meiosis.
Explain the changes in amount on DNA shown
on the meiosis graph below.
INTERPHASE
A
MEIOSIS 1: homologous chromosomes pulled to each pole
B
MEIOSIS 2: chromatids pulled to each pole
C
Half the amount of original DNA
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
It creates variation!!
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Why is variation important for
evolution?
• Some have better adapted characteristics than
others for a certain environment.
• Natural selection occurs / survival of fittest.
• Eventually gene pool will change so much that
a new species evolves.
Variation due to Crossing Over in prophase 1
• Chiasma- point of crossing (Chiasmata – plural)
• Genetic recombination
Variation due to Independent segregation
In ANAPHASE 1 the
chromosomes can be
segregated (assorted in any
random combination)
Variation due to Random Fertilisation
Gametes are
genetically
different as a
result of different
combinations of
maternal and
paternal
chromosomes
Learning Outcomes
• Describe what homologous chromosomes are.
• Identify the stages of meiosis and describe
them.
• Explain the significance of meiosis in sexual
reproduction.
• Explain the mechanisms of genetic variation
by crossing over and independent assortment.
Practice Exam Questions!!
p146, Application Questions