which cell d... - Why Zebrafis... (2024)

Hello again Shikha,
Both mitosis and meiosis are types of cell division that involve the segregation of chromosomes into daughter cells. However, there are many important differences between these two cellular processes.

Mitosis is considered an “equational” form of cell division — it occurs in cells that do not produce gametes (e.g., somatic cells). During mitosis, a cell divides once to produce two daughter cells with genetic material identical to that of the original parent cell and to each other. Both haploid and diploid cells can undergo mitosis. When a haploid cell undergoes mitosis, it produces two genetically identical haploid daughter cells; when a diploid cell undergoes mitosis, it produces two genetically identical diploid daughter cells. As we mentioned in our previous answer, mitosis is divided into five phases: prophase, metaphase, anaphase, telophase, and cytokinesis.

In contrast, meiosis is considered a “reductional” form of cell division that occurs in diploid germ cells. During meiosis, a diploid germ cell undergoes two cell divisions to produce four haploid gamete cells (e.g., egg or sperm cells), which are genetically distinct from the original parent cell and contain half as many chromosomes. The two meiotic divisions are conveniently referred to as meiosis I and meiosis II, and they are subdivided divided into different phases. Meiosis I consists of prophase I, metaphase I, anaphase I, telophase I, and interkinesis; meiosis II consists of prophase II, metaphase II, anaphase II, telophase II, and cytokinesis.

The quick answer to your question is that meiosis, with its two divisions, requires more energy than mitosis. Now, let’s briefly focus on some of the key differences between mitosis and meiosis. As we mentioned, mitosis is an equational form of cell division, and meiosis is a reductional form of cell division. Meiosis I is the reductional division step during which the number of chromosomes is reduced by half, and meiosis II is an equational division step that resembles mitosis. During prophase I of meiosis I, hom*ologous chromosomes (i.e., the two copies of each chromosome inherited from each parent) pair with each other and recombine by crossing over, leading to new combinations of alleles. Meiosis produces haploid cells with new allele combinations different from those of either parent thanks in large part to the events that occur during meiosis I.

We’ve provided a series of links below to articles that provide an in-depth view of the steps involved in mitosis and meiosis. We encourage you to focus in particular on the events that occur during prophase I. You might also be interested to read that mistakes occurring during mitosis and meiosis are linked to cancer and chromosomal abnormalities (e.g., trisomy 21) in humans, respectively.

To learn more about mitosis, follow these links:

http://www.nature.com/scitable/course-content/essentials-of-genetics-8/104

http://www.nature.com/scitable/course-content/essentials-of-genetics-8/105

http://www.nature.com/scitable/topicpage/mitosis-and-nbsp-cell-division-205

http://www.nature.com/scitable/topicpage/chromosome-segregation-in-mitosis-the-role-of-242

http://www.nature.com/scitable/topicpage/mitosis-meiosis-and-inheritance-476

http://www.nature.com/scitable/content/mitosis-a-history-of-division-12702

For more information about the coordinated events occurring during meiosis, check out the following links:

http://www.nature.com/scitable/course-content/essentials-of-genetics-8/106

http://www.nature.com/scitable/topicpage/DNA-Is-Constantly-Changing-through-the-Process-6524876

http://www.nature.com/scitable/topicpage/Meiosis-Genetic-Recombination-and-Sexual-Reproduction-210

http://www.nature.com/scitable/content/Meiosis-35062

http://www.nature.com/scitable/topicpage/Replication-and-Distribution-of-DNA-during-Meiosis-6524853

http://www.nature.com/scitable/topicpage/Mitosis-Meiosis-and-Inheritance-476

Finally, here are some links to animations showing the events that occur during mitosis and meiosis:

http://www.nature.com/scitable/content/meiosis-6656731

http://www.nature.com/scitable/content/mitosis-6656772

http://highered.mcgraw-hill.com/olc/dl/120074/bio17.swf

Reply From:Nature EducationSep 17, 2010 09:34AM

Hello Shikha,
Before we answer your question, let’s review the steps followed by cells when they duplicate their DNA and divide, known as the cell division cycle or cell cycle. There are four stages in the eukaroytic cell cycle: Gap 1 (G1), DNA synthesis (S), Gap 2 (G2), and Mitosis (M). Most cells spend the majority of their time in interphase, which consists of the G1, S, and G2 phases. During interphase, a cell grows, duplicates its chromosomal DNA, and prepares to divide. As its name suggests, DNA replication (i.e., synthesis) occurs during the S phase. During mitosis, a cell divides and splits its entire contents (including its chromosomes) between two daughter cells.

As you can imagine, the cell cycle is a tightly regulated process. There are checkpoints throughout the cell cycle to ensure that everything is in order at the end of each stage before the cell proceeds to the next stage. As you might imagine, a failure to properly complete one stage of the cell cycle before jumping into the next could have drastic consequences for a cell!

Although mitosis occupies only a small fraction of the entire cell cycle, it is an extremely important stage because it involves attaching the chromosomes to the mitotic spindle and precisely distributing one copy of each and every chromosome into the two resulting daughter cells. Mitosis can be further subdivided into five phases: prophase, metaphase, anaphase, telophase, and cytokinesis. If any of the proteins involved in mitosis malfunction, or if the chromosomes do not segregate correctly, the result could be failed cell division or, at the other extreme, uncontrolled cell growth (which can lead to cancer!).

As we mentioned, DNA replication occurs during the S phase of the cell cycle, before the cell enters the M phase and divides. You might be surprised to read that although the chromosomal DNA must be unwound during S phase to permit access by the DNA replication machinery, it becomes highly condensed and compacted as cells enter mitosis. Indeed, mitotic chromosomes are often hundreds to thousands times more condensed than interphase chromatin. As a result, individual interphase chromosomes are often not visible whereas mitotic chromosomes are clearly observed under the microscope. Why are chromosomes condensed during mitosis? Chromosome condensation plays a key role in the segregation of chromosomes between two daughter cells when a cell divides.

Now, let’s return to your specific question. You’d like to understand how cell metabolism occurs when DNA is unwound during S phase. Indeed, DNA replication typically does not occur at the same time as RNA transcription. So, how do cells produce the RNA they need to make metabolic proteins when DNA replication is happening? The quick answer is that cells often produce a stockpile of metabolic enzymes capable of supporting metabolism even when the chromosomal DNA is being replicated. Furthermore, the DNA is unwound at replication forks for only a brief moment in time before a complementary daughter strand of DNA is synthesized. Thus, the transcription of a given gene will only experience a momentary pause during DNA replication, which should not negatively impact the cell’s metabolic functions. As you can see, cell division is carefully regulated so that cells can maintain proper levels of metabolic activity while accurately segregating their DNA when they divide!

Follow these links to learn more about the cell division cycle, chromosome condensation, and DNA replication, follow these links:

http://www.nature.com/scitable/definition/mitosis-cell-division-47

http://www.nature.com/scitable/topicpage/mitosis-and-nbsp-cell-division-205

http://www.nature.com/scitable/course-content/essentials-of-genetics-8/105

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mboc4∂=A3169

http://www.nature.com/scitable/topicpage/Major-Molecular-Events-of-DNA-Replication-413

http://www.nature.com/scitable/topicpage/DNA-Packaging-Nucleosomes-and-Chromatin-310

Reply From:Nature EducationSep 17, 2010 09:28AM

Welcome back Sreeram,
The timing of transcription depends largely on the gene. Some genes that encode housekeeping proteins are transcribed all the time; other genes are transcribed only in response to a signal from the cell’s environment. As you likely know, RNA transcription is initiated from promoter elements that target transcription factors to the DNA template. RNA transcription is associated with a transcription bubble, which consists of an unwound template DNA that is accessible to RNA polymerases and transcription factors.

The short answer to your second question is no, transcription generally does not happen at the same time as DNA replication. In eukaryotic cells, DNA replication occurs during the S-phase of the cell cycle. During S-phase, RNA transcription and protein translation are largely limited to the production of histone mRNAs and proteins, thereby decreasing the likelihood that DNA polymerase will encounter RNA polymerase. In contrast to DNA replication, RNA transcription occurs primarily during the G1 and G2 phases of the eukaryotic cell cycle, when DNA polymerase is inactive.

You’re probably wondering why DNA replication is often disconnected from RNA transcription, both in time and space. Is this simply a way to avoid a collision between DNA polymerase and RNA polymerase? Or have cells evolved an efficient mechanism to deal with this molecular confrontation? Researchers have actively sought answers to these questions.

Olavarietta et al. developed a bacterial system that used a plasmid to evaluate the consequences of this chance encounter. They found that a collision between DNA polymerase and RNA polymerase triggered deleterious effects on the structure of the DNA template, including the formation of knots behind the replication fork. Ultimately, the cells exhibited incomplete replication of the plasmid and defective distribution of the plasmid to the daughter cells during mitosis. In other words, the cells could not recover from the collision between DNA polymerase and the transcription bubble. As it turns out, scientists have found very little evidence that this type of encounter occurs in nature, which is probably the result of the deleterious, and likely lethal, outcome.

But there are some situations when DNA polymerase is known to pause at selected sites where RNA transcription may occur simultaneously. This is known as replication fork pausing, and it occurs at evolutionarily conserved sites (called replication fork pause sites) scattered throughout prokaryotic and eukaryotic genomes. A DNA replication fork pause site is similar to a red light at an intersection that tells DNA polymerase to wait its turn. And it turns out these sites are also important for the regulation of RNA transcription termination. They are often found in the vicinity of highly transcribed regions, including tRNA genes. Researchers have also uncovered an association between replication fork pause sites and genetic recombination.

As you can see, it appears that nature has come up with a mechanism to avoid head-on collisions between DNA polymerase and RNA polymerase transcription bubbles. However, researchers have not yet identified the cellular mechanic hired to entangle the mess and fix the components when, and if, they collide.

To learn more about RNA transcription check out the following links:

http://www.nature.com/scitable/topicpage/The-Information-in-DNA-Is-Decoded-by-6524808

http://www.nature.com/scitable/topicpage/DNA-Transcription-426

Follow these links to learn more about gene regulation and when transcription occurs:

http://www.nature.com/scitable/topicpage/gene-expression-and-regulation-topic-room-28455

http://www.nature.com/scitable/topicpage/regulation-of-transcription-and-gene-expression-in-1086

For more in-depth reading about DNA replication, check out these links:

http://www.nature.com/scitable/topicpage/Cells-Can-Replicate-Their-DNA-Precisely-6524830

http://www.nature.com/scitable/topicpage/Major-Molecular-Events-of-DNA-Replication-413

http://www.nature.com/scitable/nated/article?action=showContentInPopup&contentPK=409

To read more about replication fork pause sites, see the following article:

http://www.nature.com/embor/journal/v8/n4/full/7400940.html

Reply From:Nature EducationSep 15, 2010 02:45PM

Hello Ankita,
Double-stranded DNA is a molecule composed of two strands of DNA that twist around one another to form a double helix. The two strands are held together by hydrogen bonds that form between complementary base pairs of nucleotides (A pairs with T, and C with G) on each strand. If the double helix is heated, then these hydrogen bonds break apart. The double-stranded DNA “melts” apart into single-stranded DNA, and this process is called DNA denaturation.

Now, what is DNA renaturation? It’s the opposite process! DNA renaturation occurs when the two single strands of a once double-stranded DNA molecule reassociate with one another — the complementary bases on each strand reform hydrogen bonds with one another, and the DNA double helix reforms. DNA renaturation kinetics involves studying the speed at which the DNA double helix reforms from two single strands of DNA.

How could you determine the renaturation kinetics of a given DNA sample? You could measure DNA renaturation by measuring how much ultraviolet light the sample absorbs at a wavelength of 260 nm. Single-stranded DNA absorbs around double the amount of ultraviolet light at this wavelength as double-stranded DNA. Therefore, you’d notice a decrease in ultraviolet absorption as the DNA renatured. By measuring the speed of this process, you’d be performing a DNA renaturation kinetics experiment.

Scientists can study DNA renaturation kinetics to determine whether two different DNA samples are related. For example, two strands that are completely unrelated (not complementary) cannot renature at all. If you’d like to learn more about DNA denaturation and renaturation (and see an image depicting the processes), please check out the following links:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A802#A813

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A802&rendertype=figure&id=A814

Reply From:Nature EducationSep 10, 2010 08:51AM

Hello Subhraprakash,
You are not alone — other investigators share your interest in the mechanisms underlying zebrafish scale pigmentation patterns. The pigmented stripes on adult zebrafish are the result of extensive post-embryonic remodeling events. Zebrafish stripes comprise three types of pigment cells (also called chromatophores), including black melanophores, yellow xanthophores, and silvery reflective iridiophores.

As you likely know, in order to identify genes involved in a process of interest (i.e., zebrafish stripe formation), geneticists often start by identifying mutants defective in that process (e.g., zebrafish lacking stripes, zebrafish with disorganized stripes). Research is ongoing to identify genes responsible for zebrafish stripes. Indeed, stripe formation is disrupted in zebrafish mutants that lack melanophores or xanthophores (but not iridiophores), showing that both melanophores and xanthophores (but not iridiophores) are required to form the striped pattern. Furthermore, zebrafish mutants with disorganized stripes have also been identified.

At least two classes of zebrafish striping mutants have been identified. One class of mutants exhibits pigmentation defects during early larval stages (i.e., colourless, nacre, sparse, rose, and fms mutants). The second class of mutants exhibits later defects in adult pigmentation patterns (i.e., asterix, obelix, leopard, puma, and hagoromo mutants). As you can see, zebrafish stripe formation is a complex phenotype that involves the activities of several genes.

Much remains to be learned about zebrafish stripes and scales — and the timing of your entry into this field couldn't be better! The zebrafish is a highly developed genetic and developmental model system, and available experimental resources are abundant. We’ve provided a series of helpful links below, which should help you learn more about zebrafish and the fascinating process of stripe formation. Best of luck to you with your studies!

To learn more about stripe and scale development in zebrafish, follow these links:

http://dev.biologists.org/content/130/5/817.full.pdf+html

http://dev.biologists.org/content/123/1/391.long

http://www.eb.tuebingen.mpg.de/departments/3-genetics/zebrafish/neural-crest-development/pigmentation

http://dev.biologists.org/content/130/15/3447.long

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1467640/

To learn more about the use of zebrafish as a genetic model system, including genetic screens and knockdown experiments, check out these links:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2564

http://www.nature.com/scitable/content/the-art-and-design-of-genetic-screen-20396

http://www.nature.com/scitable/content/effective-targeted-gene-knockdown-in-zebrafish-98693

Follow the links below to gain access to zebrafish databases and available resources:

http://zfin.org/cgi-bin/webdriver?MIval=aa-ZDB_home.apg

http://www.neuro.uoregon.edu/k12/FAQs.html

http://zebrafish.org/zirc/home/guide.php

http://www.sanger.ac.uk/Projects/D_rerio/

http://www.ncbi.nlm.nih.gov/genome/guide/zebrafish/

Reply From:Nature EducationSep 09, 2010 10:18AM