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.