Bio 161 - Meiosis

Meiosis is essentially

cell division that allows for sexual reproduction

Binary fission is

reproduction for prokaryotes

Mitosis is..

asexual reproduction

• great for maintaining pop. or repairing tissue

the whole point of mitosis is

to keep variability very low

• Parent cell’s genetic info coped, EXACT COPY → high fidelity, daughter cells are clones

Why is Meiosis necessary?

• allow for sexual reproduction (increases genetic variation)

• maintain chromosome number

Unlike Mitosis, Meiosis…

takes 2 different parent cells, mixes & introduces whole new daughter cell w/aspects of each genotype from each parent cell

What does “ploidy” refer to?

The number of complete sets of chromosomes in a cell.

What is a “set” of chromosomes

the complete collection of distinct chromosomes an organism needs to define its entire genetic blueprint (one genome copy).

What does 2n mean?

Diploid — two sets of chromosomes

• cell contains two complete sets of chromosomes—one set inherited from the mother and one set from the father (homologous chromosomes)

What does n mean?

Haploid - one set of chromosomes

• Contains only a single set of unpaired chromosomes

Are gametes haploid or diploid?

Haploid!!!

In humans almost all cells/somatic cells (except gametes) are

diploid = contain 2 copies of each chromosomes

• 1 copy from mom, 1 from dad

In haploid, the chromosome number n are

from both mom and dad mixed up

Human Diploid Chromosome Number (2n)

2n = 46

Human Haploid Chromosome number (n)

• n=23

• in humans, this applies only to gametes (sperm and egg cells)

In Meiosis you take haploid (gamete) cells from 1 parent and…

fuse + mix with haploid (gamete) cells from another parent to achieve diploid organisms!!

What are homologous chromosomes?

A maternal and paternal pair with the same genes but possibly different alleles.

Are homologous chromosomes identical?

No, they have the same genes but different alleles.

What are sister chromatids?

Identical copies of a single chromosome produced during DNA replication.

Are sister chromatids identical?

yes!

What connects sister chromatids?

centromere (concentrated cohesin proteins)

Does DNA replication change chromosome number

no!

Why doesn’t replication change chromosome number?

chromosome number is based on centromeres, not DNA amount

What changes after DNA replication?

amount of DNA doubles, not chromosome number

What is mitosis used for?

growth, repair, & asexual reproduction

What is meiosis used for?

production of gametes (sex cells)

How many cells does mitosis produce?

2 identical diploid cells

How many cells does meiosis produce?

4 genetically unique haploid cells

What happens to ploidy in meiosis I?

2n → n

What happens to ploidy in meiosis II?

no change in ploidy

• n → n !

BIG PICTURE: 1) Chromosomes prior to entering Meiosis

• 2n diploid cell in G1 → has maternal chromosome & paternal chromosome (homologous pair)

BIG PICTURE 2) Chromosomes after S Phase prior to entering meiosis

• replication achieves high fidelity of chromosomes in parent cell

• replicated chromosomes as sister chromatids

• STILL 2n !!!! → Doubled DNA but did not increase # of chromosomes

• enter meiosis this way

BIG PICTURE 3) Chromosomes Exiting Meiosis

• end up w/single chromosomes /haploid cells

• 1n females haploid = egg

• 1n in male cells = sperm

BIG PICTURE: Gametes from Meiosis have…

• egg (n) have one sister chromatid from mom

• sperm (n) have one sister chromatid from dad

BIG PICTURE 4) Fertilization

• gametes fuse together & achieve 2n diploid cell that can undergo mitotic division

• & maintain diploid state!

Life Cycles: Changes in Ploidy - Start off with…

• start with haploid gametes (n)

• start out as potential future organisms in context of sperm from father & egg from mother

• undergo fertilization (fuse)

Life Cycle: Changes in Ploidy - After fertilization…

• zygote (2n) diploid cells produced

• goes through millions of mitotic divisions → diploid adult (2n) reached

Life Cycle: Changes in Ploidy - Once Diploid adult is reached

• undergoes mitosis

• sexual maturity occurs to where reproductive cells/organs go through meiosis!

Life Cycle: Changes in Ploidy - After sexual maturity

• meiosis occurs to achieve haploid gametes (n)!

• cycle begins again

Meiosis: Two Chromosomal Divisions - Meiosis I

• Parent cell (2n) has sister chromatids from mom, sister chromatids from dad, has undergone replication of chromosomes

• In meiosis 1 homologous pairs separate!

→ still 1 chromosome in each daughter cell

Meiosis: Two Chromosomal Divisions - Mitosis II

• In meiosis II sister chromatids separate !

• 4 daughter cells result (each n)

• still haploid but have HALF DNA of parent cell

• each haploid cells end up w/SINGLE copy of DNA → gametes !

Summary of sexual reproduction (human wise)

Sexual reproduction uses meiosis to make haploid gametes (23 chromosomes), fertilization restores diploid number (46), and mitosis grows the organism from the zygote while meiosis in adults creates genetically unique gametes for the next generation.

Before replication

• Chromosomes: 2n

• DNA: normal

After replication (before Meiosis I)

• Chromosomes: 2n (still the same number — just duplicated)

• DNA: double

After Meiosis I

• Chromosomes: n

• DNA: original amount

After Meiosis II

• Chromosomes: n

• DNA: half of original amount

Meiosis I: What is produced by interphase prior to entering Meiosis?

• have germinal cells of individual species → give rise to gametes

• have undergone replication

Meiosis I: What is in early prophase I?

• have double amount of DNA

• 2n = 6 chromosomes (in this case)

• 3 pairs of homologous chromosomes

• chromosomes condense & spindle apparatus forms, nuclear envelope beings to break down & synapsis (paring) of homologous chromosomes

Meiosis I: What happens in late prophase I/prometaphase I?

• Chiasmata (cross over points) visible → formed by nonsister chromatids

• nuclear envelope broken down

• spindles associate with PAIRS of homologous chromosomes

Meiosis I: What happens in metaphase I?

• pairs of homologous chromosomes aligned @ plate (not sister chromatids like in meiosis)

• cross over has occurred! → pieces of homologous pairs of chromosomes from maternal & paternal, mixing, increases variability

Meiosis I: What happens in anaphase I?

• pairs of homologous chromosomes oulled apart & move to opposite poles of spindle apparatus

• Chiasma broken, centromere cohesins intact

Meiosis I: What happens in telophase I & cytokinesis?

• Homologs are at opposite poles, spindle apparatus disassembles

• Cell divides, chromosomes # reduced

• each daughter cell has double # of nucleic acid than it will as a gamete (still as sister chromatids) → unique chromosome as pair of sister chromatids (HAPLOID CELLS)

• “normal” amount of DNA

Meiosis II: What happens in Prophase II?

• cells enter as HAPLOID cells → chromosomes have sister chromatids

• spindle aparatus reforms

Meiosis II: What happens in prometphase II/ metaphase II?

• microtubules bind to sister chromatids

• sister chromatids align @ metaphase plate

Meiosis II: What happens in anaphase II?

• sister chromatids pulled apart!! move to opposite poles of spindle apparatus

Meiosis II: What happens in telophase II & cytokinesis?

• Daughter chromosomes are at opposite poles, spindle apparatus disassembles

• Cell divides, chromosome # unchanged

• end up w/4 gametes → each has single chromatid of single chromosomes → all 1n

Meiosis leads to genetic variation through…

• Crossing Over

• Random Alignment of homologs

Genetic variation 1st: Crossing Over - Condensation

2 chromosomes as homologous pair of chromosomes condense

→ sister chromatids present held together by cohesin proteins

Genetic variation 1st: Crossing Over - Pairing

break in chromatid DNA

Genetic variation 1st: Crossing Over - Synapsis

• pairing of nonsister chromatids → synaptonemal complex

• prophase I

Genetic variation 1st: Crossing Over - Partial Separation of homologs

• late prophase

• crossover between non-sister chromatids → swap pieces of DNA

Meiosis: Crossing over is…

• required for correct segregation

• important for new combinations of alleles (gene versions)

Genetic Variation: 2nd) Random Alignment of homologs

• based on how homologous chromosomes align along metaphase plate

• either only maternal or only paternal (in this case) or swapped → different combinations of chromosomes

Random Alignment of homologs - Humans have 23 pairs

2²³

→ 8 million possible combinations of chromosomes

What is independent assortment?

Random alignment of homologous chromosomes in metaphase I.

What does independent assortment create variation in?

Different combinations of chromosomes.

What is crossing over?

Exchange of DNA between homologous chromosomes in Prophase I.

What does crossing over create variation in?

Genetic variation within chromosomes. (different alleles)

Formula for chromosome combinations from independent assortment?

2ⁿ

How are chromosomes arranged in mitosis metaphase?

Single chromosomes line up individually.

How are chromosomes arranged in meiosis I metaphase?

Homologous pairs line up together.

Key difference between meiosis I and mitosis?

Meiosis I separates homologs; mitosis separates sister chromatids.

What does a karyotype show?

Number, size, shape, and arrangement of chromosomes.

• sister chromatids not visible in karyotype

What is aneuploidy?

Abnormal number of individual chromosomes

Mistakes in Meiosis: Change in Chromosomes Number - Trisomy

• example of aneuploidy

• Trisomy 21 = down syndrome (in humans)

If cell went through interphase, the # of total individual chromatids present in the cell this karyotype was taken from is equal to?

94! (or # total DNA pieces present)

Mistakes in Meiosis: Nondisjunction

Failure of homologous chromosomes to separate properly.

• microtubules pull innappropiate # oh homologs

• results in aneuploidy

When can nondisjunction occur?

Meiosis I or Meiosis II

Nondisjunction in Meiosis I results in what gametes?

All abnormal (n+1 or n-1)

Aneuploid n+1

2n + 1 (trisomy)

• many nonproductive

• trisomy 21 is rare case of productive

Aneuploid n -1

2n-1

• extremely rare

• usually nonproductive for non sex chromosomes (only sex chromosomes ok w/1 copy)

What does an unreplicated chromosome look like?

single rod

What does a replicated chromosome look like?

X shaped structure w/two sister chromatids

What does a homologous pair look like?

Two X-shaped chromosomes together. (after replication)

Why is meiosis important for evolution?

It creates genetic diversity in offspring.

Turner syndrome is an example of

• X chromosome monosomy

• a genetic condition affecting only females. It occurs when one of the X chromosomes is completely or partially missing

• Because embryos with a 45,Y chromosomal makeup are not viable early in development, Turner syndrome only occurs in individuals with a female chromosomal background.

Autosomal chromosomes =

non-sex chromosomes

• need 2 copies of autosomal chromosomes