Meiosis and Errors in Meiosis Quiz

Label a chromosome

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How does meiosis result in daughter cells with half the number of chromosomes of the parent cell?

This process results in half the number of chromosomes of the parent cell because the chromosomes are copied once, but are divided twice (meiosis I separate homologous chromosomes and meiosis II separate sister chromatids)

Explain the process of meiosis I

**Interphase**: Chromosomes are ***replicated*** to form ***sister*** ***chromatids***, which are ***genetically*** ***identical*** and join at the ***centromere***. The ***single*** ***centriole*** is also ***replicated***.

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__**PMAT**__:

**Prophase 1**: During ***synapsis***, chromosomes ***condense*** and homologous chromosomes ***form pairs*** to create ***tetrads***. This is also when ***crossing over*** can occur in the chromatids of homologous chromosomes. A ***spindle*** forms from each ***centromere*** and ***spindle fibres*** move the ***tetrads*** around.

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**Metaphase 1**: The ***tetrads*** are all arranged at the ***metaphase plate.***

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**Anaphase 1**: The ***homologous chromosomes*** are ***separated*** and are pulled to ***opposite ends*** of the cell.

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**Telophase 1**: Movement of the ***homologous chromosomes*** continues until a ***haploid set*** is at ***each pole.***

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**Cytokinesis 1**: Two **haploid daughter cells** form. **Chromosomes** are still **double-stranded.**

Explain the process of meiosis II

\*Note: Meiosis II is **very similar** to mitosis, but **not identical**

__**PMAT:**__

**Prophase 2:** ***Spindle fibres*** form, attaching to ***each sister chromatid*** and ***moving*** them around.

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**Metaphase 2:** ***Sister chromatids*** are arranged at the ***metaphase plate*** and the ***sister chromatids*** face ***opposite ends.***

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**Anaphase 2:** The ***centromeres*** of ***sister chromatids*** ***separate*** and the now separated sisters ***travel toward*** ***opposite ends*** of the ***cell***.

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**Telophase 2:** The ***separated sister chromatids*** arrive at the ***opposite ends*** and ***nuclei*** form around the ***chromatids***.

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**Cytokinesis 2:** ***Cytokinesis*** separates the ***cytoplasm*** and ***4 haploid daughter cells*** are produced.

How are mitosis and meiosis similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of daughter cells?

Both of these processes are similar in the way that the chromosomes separate at opposite ends of the cell.

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However, they also differ in the number of cells produced as mitosis produces 2 diploid, identical daughter cells, whereas, meiosis produces 4 haploid, non-identical daughter cells.

What is nondisjunction?

It is when there are problems with the meiotic spindle causing errors in daughter cells, resulting in too many or too few chromosomes.

What is crossing over?

It is when the non-sister chromatids of homologous chromosomes exchange segments of their DNA. This occurs during synapsis in prophase 1 of meiosis.

What is independent assortment?

It is when homologous pairs line up on the equator of the cell independently of each other. This occurs during metaphase 1 of meiosis.

How do processes like crossing over and independent assortment increase genetic variation?

What is a gene?

A gene is a section of DNA that encodes for a certain trait e.g. eye colour

A part of a chromosome that controls the expression of a trait and is passed on to offspring; it has a specific DNA sequence

What is a chromosome?

It is a structure in the nucleus that contains DNA

What are homologous chromosomes?

It is a pair of chromosomes that contain the genetic code for the same traits. One chromosome comes from each parent. e.g. chromosomes contain information about hair colour

What is an allele?

An allele is a variant form of a gene e.g. blue eyes

What is the difference between a gene and an allele?

Genes encode for a certain trait, whereas, alleles are variant forms of a gene

e.g. a gene for hair colour, an allele for brown hair

How do DNA, genes, chromosomes and alleles account for the transmission of hereditary characteristics?

What is nondisjunction?

It is when a gamete with the incorrect number of chromosomes is fertilized, causing errors in daughter cells

e.g.

homologous chromosomes not separating properly during meiosis I

sister chromatids failing to separate during meiosis II

too many or too few chromosomes

How does nondisjunction result in the wrong number of chromosomes in a gamete?

What are some examples of human genetic disorders that can result from a gamete with the wrong number of chromosomes combining with a gamete with the correct number of chromosomes?

Down syndrome (trisomy 21), Turner syndrome (monosomy X or XO), Klinefelter’s syndrome (XXY male), Jacob’s syndrome (XYY male), trisomy X (XXX - doesn’t actually cause any health problems, just a female with an extra X chromosome)

What is down syndrome (trisomy 21)?

3 copies of chromosome 21 are made instead of 2

1 in 700 children born in the U.S. have trisomy 21

Chromosome 21 is the smallest human chromosome, but still has severe effects

Correlates to the age of the parents, as the mother and father grow older, the likelihood of trisomy 21 in a baby increases

What is Turner syndrome (monosomy X or XO)?

It is a female-only genetic disorder that occurs when an individual only has one X chromosome instead of 2

1 in every 5000 births

Degree of effects are varied: webbed neck, short stature, sterile

What is Klinefelter’s syndrome (XXY male)?

It is when a male is born with an extra X chromosome

1 in every 2000 births

Have male sex organs, but are sterile

Feminine characteristics: some breast development and lack of facial hair

Tall

Normal intelligence

What is Jacob’s syndrome (XYY male)?

It is when a male receives an extra Y chromosome from his father

1 in 1000 births

Slightly taller than average

More active

Normal intelligence or slight learning disabilities

Delayed emotional immaturity

Normal sexual development

What is trisomy X (XXX)?

It is when a female is born with an extra chromosome

1 in every 2000 births

Doesn’t have any negative effects and produces healthy females