[WEEK2] Part2 Meiosis

LEARNING OBJECTIVES

Explain why humans resemble their parents but are not identical to them

They receive genes from each of their parents, so they resemble to their parents. However, during meiosis chromosomes from each parents align randomly in metaphase 1 and crossing over happens during prophase1. This makes the variation in genes in gametes. And also the millions of gametes fertilise so the odds make the egg variations.

 

Describe how the chromosome number changes throughout the human life cycle

Chromosome number is count by the number of centromeres. During mitosis, chromosome number doesn't change and remains 46. (22 automosomes and 2 sex chromosomes). However, during meiosis, 46 chromosomes are splited into two daughter cells in meiosis 1 by 23 chromosomes, and at the end of meiosis1, each gametes own 23 chromosomes.

 

Describe the concept of ploidy

It describes the set and the number of chromosomes. It differ by the species. 2n (diploid) expresses that homologous chromosomes are exisiting. Homologous chromosomes have almost no difference in their size and the location of the gene (allele).

 

Explain karyotyping and describe how chromosomes are recognised

Dying, and coloring shows the gene 

Compare and contrast sister chromatids to homologous chromosomes

 

 

Explain the phases of meiosis and describe the events characteristic of each phase

 

 

Recognize the phases of meiosis from diagrams and micrographs

 

 

Define synapsis, crossing over, tetrad, chiasmata and at which phase of meiosis these occur

 

 

Distinguish between mitosis and meiosis    

 

 

Explain how independent assortment, crossing over, and random fertilisation contribute to genetic variation in sexually reproducing organisms

 

 

Explain the evolutionary significance of genetic variation

 

 

Explain the chromosomal basis of trisomy 13 (Patau syndrome) and describe how and when this chromosomal abnormality could occur. 

 

 

 

 

PPT

 

 

 

 

 

 

What is meiosis used for?

 

 

 

 

 

 

 

 

 

 

The process of Meiosis

*Cells arrested when the chromosomes are most highly condensed - at metaphase - are stained and then viewed with a microscope equipped with a digital camera

*Homologous chromosome has similarity in the size, position of the centromere, and pattern of stained bands.

*Heterochromatic - stained darkly, euchromatin - stained lightly

*automosome : 상염색체

*sex chromosome : 성염색체

*homologous chromosome : 상동염색체 = homologs

 

 A gene is a functional unit of DNA, and your DNA is organized onto chromosomes. Chromosome banding is a little like tie-dying your chromosomes.

A chromosome is a unit of tightly-packed DNA. DNA has to wrap tightly around itself, because you have quite a lot of it. In fact, if you unrolled all the DNA in a single one of your cells, it would be about three meters long. Humans have 46 chromosomes - 23 from Mom and 23 from Dad.

In chromosome banding, we treat chromosomes with chemicals to stain them and learn about a chromosome by how it stains. There are several different types of stains we can use.

• G-banding uses a stain called Giemsa stain. G-banding gives you a series of light and dark stripes along the length of the chromosome. We will discuss G-banding in the most detail because you will likely see G-banding if you take a genetics class.
• Q-banding uses a stain called quinacrine. Q-banding yields a fluorescent pattern. It is similar in pattern to G-banding but glows yellow.
• C-banding only stains the centromeres. Centromeres are little constricted portions of chromosomes. That's where sister chromatids (two copies of the same chromosome) will attach to each other when the cell is getting ready to divide.
• R-banding is the opposite of G-banding. R-banding stains complementary regions to those stained with G-banding and they are used together to determine chromosomal deletions.

 

 

 

-Ploidy refers to the number of chromosome sets

(diploid, haploid)

-The haploid gametes are replicated into diploid and generate haploid daugther cells

-The haploid gametes (sperm and egg) fertilised and become diploid (2n) (zygote)

 

 

 

 

 

 

 

 

 

 

The process of Meiosis

*Homologs have different versions of genes, each called allele(대립유전자), at corresponding loci(유전자 자리)

https://www.youtube.com/watch?v=VzDMG7ke69g 

 

 

 

 

 

 

 

 

Meiosis vs mitosis 

-Chiasma : site of crossing over

-Duplicated homologous chromosomes cross over by chiasma in the prophase 1

 

 

 

 

 

 

 

 

 

Sources of variation

1. Independent assortment of maternal, paternal chromosomes (Metaphase1)

2. Crossing over during prophase (Prophase1)

3. Random union of gametes (When fertilised)

 

 

 

 

 

 

 

 

 

 

 

When it goes wrong

https://pubmed.ncbi.nlm.nih.gov/15266400/

Trisomy 13 : karyotype 47, XX + 13

-Consider the human lifecycle and explain at which stages this chromosomal abnormality could occur 

: 13 homologous chromosome is not divided in metaphase 1

 

 

 

 

 

 

 

 

 

 

SELF PACED QUIZ

 

1.

-Mitosis is used for cell growth, cell repair  / Meiosis is used for producing gametes (sperms and eggs) 

-Mitosis produces 2 genetically identical daughter cells / Meiosis produces 4 unidentical daughter cells

-Mitosis finishes by one phase and ends up with diploid / Meiosis has 2 phase (Meiosis1, and Meiosis 2) and ends up with haploid

-During Metaphase 1, each of the homologous chromosome is aligned in the metaphase plate in mitosis / Homologous chromosome (tetrads) aligned in the metaphase plate in meiosis. 

-During anaphase 1, sister chromatid is seperated in mitosis / Homologous choromosome is seperated in meiosis

2.

There are three elements that attribute to increasing the frequency of genetic variation

1) Maternal and Paternal chromosome in each set of homologous chromosome is aligned randomly in metaphase1

2) Crossing over occurs during the prophase1 when they form a tetrads 

(crossing over is the exchange of DNA with nonsister chromatids)

3) Zygotes are fertilised randomly 

 

3.

Karyotype shows the number, size and shapes of the entire chromosomes. Human male has 46 chromosomes with 22 pairs of autosomes and X,Y sex chromosomes.

4.

Mitosis is for cell growth, repair and development of zygote when it is fertilised.

Meiosis is for producing sex cells. Because they produce haploid cells, it allows to make diploid zygote when it is fertilised with other sex cell. Also, variations exists in meiosis which make the offspring unique with their parents

5.

Crossing over happens with nonsister chromatid. However, during anaphase1 tetrads are divided so the meiosis 2 misses homologous chromosome. There is only one of the homologous chromosome.