Chromosomes in Human Cells

     Every cell in humans besides gametes are called somatic cells, which each have 46 chromosomes.  These chromosomes can only be distinguished from one another by microscopic examination.  This examination reveals that there are 2 chromosomes of each type.  One type is homologous chromosomes:  they have the same length, centromere position, and staining pattern.  Both chromosomes carry genes controlling the same inherited characteristics.  The other type of chromosome is the sex chromosome.  These are referred to as X and Y.  Human females have a homologous pair of X chromosomes and males have one X and one Y. (Campbell p.240) (Riley)

     The 46 chromosomes that humans possess are a result of the inheritance of 2 sets of chromosomes from each parent.  The maternal set has 23 chromosomes and the paternal set has 23 chromosomes.  Any cell that has 2 sets of chromosomes is called a diploid cell and has a diploid number of chromosomes (2n).  Humans have a diploid number of 46, which is the number of chromosomes in our somatic cells.  The sex cells, also known as gametes, only have a single chromosome set.  These cells are called haploid cells and have a haploid number of n chromosomes.  Humans have a haploid number of 23.  This set of chromosomes contains the 22 autosomes plus a sex chromosome. (Campbell p.241) (Riley) 

 

(www.biologycorner.com) Picture of a chromosome that has some labeling of the parts. (Kaufman)

The Stages of Meiosis

     Interphase

• Chromosomes replicate
• Each replicated chromosome contains 2 genetically identical sister chromatids connected at the centromere
• Centrosome replicates, forming 2 centrosomes

     Prophase I

• Takes up more than 90% of time
• Chromosomes condense
• Homologous chromosomes pair along their lengths
• DNA molecules in nonsister chromatids break at corresponding places and then rejoin during crossing over
• In synapsis, a protein structure forms between homologues holding them tightly together
• Protein disassembles and each chromosome pair is seen as a group of 4 chromatids
• Nuclear envelope begins to break down
• The homologous pairs move toward the metaphase plate (Campbell p.244) (Riley)

     Metaphase I

• Pairs of homologous chromosomes are arranged on the metaphase plate
• Both chromatids of a homologue are attached to kinetochore microtubules from one pole

     Anaphase I

•      Chromosomes move toward the poles
• Sister chromatids remain attached at the centromere and move toward the same pole
• Homologous chromosomes move toward opposite poles

     Telophase I and Cytokinesis

• Cytokinesis forms 2 haploid duaghter cells
• In animal cells a cleavage furrow is formed
• In some cells the nuclear envelope and nucleoli re-form
• No chromosome replication occurs

     Prophase II

•  A spindle apparatus forms
• Chromosomes composed of 2 chromatids move toward the metaphase II plate  

     Metaphase II

•  Chromosomes are positioned on the metaphase plate
• The kinetochores of sister chromatids are attached to microtubules extending from opposite poles

     Anaphase II

• Centrosomes of each chromosome finally separate and the sister chromatids come apart
• Sister chromatids move individually towards opposite poles

     Telophase II and Cytokinesis

• Nuclei form
• Meiotic division of 1 parent cell produces 4 daughter cells
• Each of the 4 daughter cells is genetically distinct from the others and from the parent cell (Campbell p.245) (Riley)

 

Crossing Over

     In the early part of prophase I, two nonsister chromatids, one from mom and one from dad, break at the same place in their DNA and then rejoin the other chromatid where it was broken off. Normally, a human chromosome can expect to go through crossing over 1-3 times depending on its size and position of the centromere. Crossing over allows us to get a mix of both parents genes and thus leads to genetic variation (campbell 248). (Ramey)

 Video: http://www.youtube.com/watch?v=MqaJqLL49a0&feature=related (Kaufman)

http://media-2.web.britannica.com/eb-media/88/78588-036-DAFF2140.jpg (Brandenburg)

 

 

Summary of the Stages of Meiosis

     Meiosis, unlike mitosis, has 2 consecutive cell divisions called meiosis I and meiosis II.  These divisions result in 4 daughter cells that each have half as many chromosomes as the parent cell.  Mitosis only produce 2 daughter cells.  In meiosis, a single homologous pair of chromosomes in a diploid cell is replicated and copied into 4 daugher haploid cells.  After the chromosomes replicate during interphase, the homologous chromosomes separate during meiosis I.  Then during meiosis II, the sister chromatids separate, producing 4 daughter cells.  The diploid cell must divide twice during meiosis in order to produce the haploid daughter cells. (Campbell p.243) (Riley) 

 

ViDEO: http://www.youtube.com/watch?v=SMve1h4Wlbo (Friedman)

 

Importance of Meiosis. (Awan)

If human gametes were to go through mitosis, they would be diploid like somatic cells. Recall that meiosis reduces the number of chrmosomes from diploid to haploid. Under mitotic conditions, the 2 gametes that have fused will double the chromosome number to 92, and each subsequent generation would double the number of chromosomes again and again. This will lead to no genetic variation and eventually the extinction of the human species. (Awan)

 

Genetic Variation (Awan)

There are three mechanisms that contruibute to genetic variation: independent assortment, crossing over, and random fertilization

    I. Independent assortment: Chromesomes line up at the metaphase plate in Metaphase I of meiosis. They have a 50% chance of lining up at each side.

   II. Crossing Over: produces recombinant chromosomes, individual chromosomes that carry genes derived from two different parents. Due to this there is a wider variety of traits that get passed from parents to offspring. This event happens in the prophase I stage of meiosis.

       - Recombinant chromosomes individual chromosomes that carry genes (DNA) derived from two different parents.

  III. Random Fertilization: Each male and female gamete represents one of approximately 8 million possible chromosome combinations..

      the fusion of a single male gamete and a single female gamete during fertilization will produce a zygote with any of about 64 trillion diploid combinations.(Awan)(Wheeler)

 

 

 

Advantages/ Disadvantages of Sexual Reproduction

 

 Offspring produced by sexual reproduction will have a modified DNA of the parents.  The new DNA produced may be an improvement to that of the parent.

Ex. A specie with alot of genetic variation is much less susceptible to any one kind of attack (disease or predator).

 Parents reproducing by sexual reproduction are, also, more likely to produce offspring that are able to live in environments different from their own.  This process is called Darwinism, which is a process where the individuals that are able to survive in an environment better than others of the same species will produce offspring that can live in that environment. 

Ex.  A white rabbit is more likely to live in an area covered in snow than a brown rabbit, thus being able to produce more offspring that are more genetically suitable for that environment.     (Archibald)

 

The disadvantages to sexual reproduction is that you need two parents to produce an offspring in many cases is time consuming to find the right person or mate.

Good gene combination could be lost.

Ex. Snake in the wild needs camouflage to blend in to the surroundings to hunt or protect itself.  If a snake produced is albino then the snake has lost all advantages to blend in to its surroundings.  (collins)

 

 Advantages/ Disadvantages to Asexual Reproduction

 

Advantages of asexual reproduction are that only one parent is needed

The offspring are identical to the parent

Good genetic traits are conserved and passed to the offspring 

Less energy is needed for asexual reproduction.  (Archibald)

Disadvantages are that no new DNA combination is acquired. (collins)

 Species using asexual reproduction are unable to adapt to new environments.

Bad genetic traits are conserved.  (Archibald)

 

Another form of asexual production is called budding. Budding  is used by hydra and jellyfish. This is when a localized mass of cells divides by mitosis on the the parent organism and eventually detatches itself when it has grown large enough. These buds are exact clones to their parents. Also, while a bud is being produced, it is taking away nutrients from the parent organism (campbell and reece 239). (ramey)

 

Similarities/Differences of Mitosis vs. Meiosis

• mitosis-synapsis of homologous chromosomes
• meiosis-synapsis of homologous chromosomes occurs during prophase I
• mitosis-DNA replication occurs during interphase before mitosis begins
• meiosis-DNA replication occurs during interphase before meiosis I begins
• mitosis-number of cell divisions=1
• meiosis-number of cell divisions=2
• mitosis-number of daughter cells produced=2 identical diploid cells
• meiosis-number of daughter cells produced=4 haploid cells that are different from the parent cell (Komis)

 

Alternation of Generations

 

    The alternation of generations arises due to mitotic and meiotic processes in every sexually reproducing organism. In humans, a haploid cell is reproduced through meiosis. When a haploid cell is fertilized, it creates a zygote, which then undergoes mitosis in order to grow. The gametes in the zygote will eventually go through meiosis.

       In plants and some algae, a diploid sporophyte undergoes meiosis to produce haploid spores. The spores divide through mitosis, creating haploid gametophytes, which then reproduce mitotically to form gametes. These gametes are fertilized, and the process countinues.

            In fungi, some protists, and some algae, a diploid zygote forms, but it divides meiotically without creating gametes. Haploids are formed, which then divide mitotically to produce a haploid multicellular adult organism. (Campbell 242) (Brose)

 

Evolutionary Significance of Genetic Variation Withing Populations (Awan)

Charles Darwin recognized that a population evoloves throught differential reproductive success of its variant membranes. Individuals that are best suityed to the environment are the ones that will leave the most offspring, so leaving the most genes. As the environment changes, the population may survive if some members of the same generation can cope with the new conditions.(Awan)

 

 

 

 Practice Questions

1. A cell with a diploid number of 6 could produce gametes with how many different combinations of maternal and paternal chromosomes?

a. 6

b. 8

c. 12

d. 64

e. 28

2. in most fungi and some protist,

a. the zygote is the only haploid stage.

b. gametes are formed by meiiosis.

c. the multicellular organism is haploid.

d. the gametophyte generation produces gametes by mitosis

e. reproduction is exclusively asexual.

 3.Which of the following would not be considered a haploid cell?

a. daughter cell after meiosisII

b. gamete

c. daughter cell after mitosis in gametophyte generation of a plant

d. cell in prophase I

e. cell in prophase II

4. Meiosis II is similar to mitosis because

a. sister chromatids separate.

b. homologous chromosomes separate.

c. DNA replication precedes the division.

d. they both take the same amount of time.

e. haploid cells are produced

 

answers: 1. b 2. c 3. d 4. a (Komis)

 

Works Cited

Cambell, Neil A. and Jane B. Reece.  AP Edition: Biology, 7th Edition.  San Fransisco: Pearson: Benjamin Cummings, 2005.

Campbell Neil A and Jane B Reece. AP edition: Biology 7th edition. San Francisco Pearson Benjamin Cummins 2005. pgs: 247-248

Campbell Neil A and Jane B Reece. AP Edition: Biology 7th edition. San Francisco Benjamin Cummins 2005. Pgs 248-249, 241, 242