Topic 4 Meiosis To learn more about the cell cycle
• A second type of cell division designed to make haploid (n) cells called gametes– Gametes include sperm (from testes) and egg
(from ovaries)– 2n→n
• Meiosis occurs in 2 stages to produce a total of 4 haploid cells
Why are haploid cells necessary?
So that when 2 haploid gametes fertilize, the embryo has the correct number of chromosomes
Mitosis (IPMAT)
Interphase Prophase
Metaphase AnaphaseTelophase
Identical cells are produced in mitosis.
2n 2n
Compare mitosis to meiosis
Synapsis (joining of homologous chromosomes)
and crossing over allow pieces of DNA to be exchanged between
chromatids. This introduces genetic variation. The daughter cells are not genetically identical in
meiosis.
Homologous chromosomes
Crossing Over
Crossing over provides an important advantage to sexual reproduction. With increased genetic diversity, a population is less susceptible to disease.
Gametogenesis
• Meiosis in humans in also known as gametogenesis
• In males, sperm production is known as spermatogenesis and occurs in the testes– Each time, 4 haploid sperm are produced
• In females, egg production is known as oogenesis and occurs in the ovaries– Each time, 1 haploid egg and 3 polar
bodies are produced
Girl 2n = 46
Boy2n = 46
Eggn = 23Meiosis
Spermn = 23
Meiosis
Fertilization
Zygote2n = 46 Child
2n = 46
Mitosis
Mitosis
Make sure you know all the processes (blue) and all the types of cells produced (green)
Practice: List as many differences as you can between mitosis and meiosis
• Occurs all over the body (all somatic cells)
• Occurs only in the gonads (ovaries and testes)
• Used for growth, maintenance and repair
• Used for gamete production (sperm and egg)
• 2n 2n (46 chromosomes 46 chromosomes) • 2n n (46 chromosomes 23 chromosomes)
• 2 identical cells are produced • 4 haploid cells are produced• Crossing over occurs leading to
genetic variation
Topic 4-2: Mutations
• Mutations (changes in DNA) can have adverse effects or be beneficial
• A mutation is a mistake in replication that happens naturally or are caused by outside factors
– Ex. Chemicals or radiation • Cells can usually repair small
mistakes and a mutation does not affect an organism.
• However, sometimes mutations can cause the genes to stop working or to function differently.
• Mutations can be passed on to future generations
Beneficial MutationsSometimes mutations can be beneficial to an organism. When the mutation gives an organism an advantage it is more likely to survive. The fittest individuals survive and pass on their traits to their offspring.
Mutations
ChromosomalChanges in structure or number of
chromosomes
Point/Gene mutationChanges in part of a DNA strand
FrameshiftAddition or deletion
of nitrogen bases to a DNA sequence
Non-FrameshiftSubstitution of one or morenitrogen bases for another
Chromosomal Mutations• Chromosomal mutations can be diagnosed by a karyotype• In karyotypes, chromosomes are organized according to 3
characteristics– Size, banding pattern, centromere position
Gender?
These chromosomes are all taken from ____ cell2n
Detecting Genetic Disorders – how are cells collected to make a karyotype?
Ultrasound locates position of fetus
Amniocentesis - uses a needle to withdraw some amniotic fluid from around fetus and then fetal cells can be analyzed
Chorionic Villus Sampling (CVS) – draws cells from outer membrane of embryo and can be used as early as 8 weeks of pregnancy
Examples of chromosomal mutations
Down Syndrome (Trisomy 21)
Turner’s Syndrome (XO)
Jacob’s Syndrome (XYY)
Examples of chromosomal mutations
Klinefelter’s Syndrome (XXY)
Point/Gene Mutations – Frameshift and non-frameshift
• Frameshift mutations include adding or deleting 1 or 2 bases to a DNA strand • Think of someone jumping in front of you or getting out of line – this means
that anyone after will be affected • Non-frameshift mutations include substitutions where 1 or 2 bases
in a DNA strand are replaced by another • Think of a substitute teacher or player
Original DNA strand: … TCG ATA GCC CA …
Addition … TCA GAT AGC CCA …
Substitution
Deletion … TCG AAG CCC A …
… TCG ATA TCC CA …
Frameshift mutations often have more
severe consequences since it changes the reading frame, like
changing the way we read a sentence
Practice Identify the mutation type as frameshift or non-frameshift as well as a substitution, deletion or addition (first strand is unmutated)
Only one nitrogen base changed, therefore it is a substitution (non-frameshift)
After the change (blue) everything shifts backwards, therefore it is a deletion (frameshift)
Point/Gene Mutations – Frameshift and non-frameshift
• Point mutations can affect the amino acids and proteins produced
Original DNA strand:
Original amino acids:
… ATG CGA GTT CA …
Affected amino acids:
MutatedDNA strand: … CTG CGA GTT CA … … TGC GAG TTC A … ..ACT GCG AGT TCA..
substitution frameshift
Methionine – Arginine – Valine
Leucine – Arginine - Valine
Only 1 amino acid is affected
Cysteine – Glutamine – Phenylalanine
Deletion
Threonine – Alanine – Serine
Addition
Many amino acids are affected
Asparagine – Leucine – Valine
AAC TTT GTA
Asparagine – Phenylalanine – Valine
Antibiotic Resistance• Bacterial infections are usually treated
with antibiotics• However, over time bacteria may become
resistant • There are 3 ways this can occur:
1. Mutation and natural selection2. Transformation of DNA fragments 3. Transformation of plasmids
Mutations and Natural Selection
Random mutations can occur in genes that make bacteria resistant to certain antibiotics.
When the bacteria divide, their resistant genes are passed on. The bacteria that are resistant to antibiotics are more likely to survive.
Transformation of DNA Fragments
Bacteria can pick up pieces of DNA from their environment. The new pieces of DNA get added to the bacterial chromosome. The bacterium can now make the proteins from the new pieces of DNA. This process is called transformation.
Transformation of Plasmids
They have genes that are not necessary for the regular function of the cell but often have extra genes like antibiotic resistance. When bacteria come into contact with each other, they can exchange plasmids. This means that the genes for antibacterial resistance can be shared.
Bacteria have special small circular pieces of DNA called plasmids.
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