How is DNA replicated different during meiosis?

In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II at the same time. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

Prophase II

If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

Prometaphase II

The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

Metaphase II

The sister chromatids are maximally condensed and aligned at the equator of the cell.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles (Figure 1). Non-kinetochore microtubules elongate the cell.

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell. During meiosis II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

Figure 1 In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid and have only one copy of the single set of chromosomes. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover.

The entire process of meiosis is outlined in Figure 2.

Figure 2 An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.

Meiosis II begins with the 2 haploid cells where each chromosome is made up of two connected sister chromatids. DNA replication does NOT occur at the beginning of meiosis II. The sister chromatids are separated, producing 4 genetically different haploid cells.

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

OpenStax, Biology. OpenStax CNX. May 27, 2016http://cnx.org/contents/[email protected]:1Q8z96mT@4/Meiosis

DNA is replicated during the S phase of interphase, resulting in chromosomes that contain two identical DNA strands

• These genetically identical strands are called sister chromatids and are held together by a central region called the centromere
• These chromatids separate during meiosis II, becoming independent chromosomes each made of a single DNA strand

If DNA replication did not occur prior to meiosis there would be no need for a 2nd meiotic division (meiosis I = diploid → haploid)

• The fact that DNA replication does occur suggests that meiosis evolved from mitosis (where initial DNA replication is necessary)
• One benefit of the duplication of chromatids is that it increases the potential for genetic recombination to occur (more variation)

  Cells must replicate their DNA before they can divide. This ensures that each daughter cell gets a copy of the genome, and therefore, successful inheritance of genetic traits. DNA replication is an essential process and the basic mechanism is conserved in all organisms. DNA replicates in the S phase of the cell cycle and initiates at specific regions in the DNA sequence known as DNA replication ‘origins’. A number of proteins participate in DNA replication and the process is subject to scrutiny by cell surveillance mechanisms called cell cycle checkpoints. These checkpoints ensure that replication of DNA occurs just once per cell cycle. Defects in DNA replication could give rise to damaging mutations including those that cause cancer.

  The Initiation of DNA replication

  The stage for DNA replication is set in the G1 phase of the cell cycle and DNA is synthesized in the S phase. DNA replication is initiated at specific sites in the genome known as the ‘origins’ which are recognized and bound by origin binding proteins. Replication commences at a single origin in prokaryotes and at multiple origins in eukaryotes, however, the basic mechanism of replication is conserved in all organisms . In eukaryotes, initiator proteins ORC, Cdc6 and Cdt1 recruit the replicative helicase . The eukaryotic replicative helicase is a complex of proteins called the CMG helicase consisting of Cdc45, Mcm2-7 and GINS proteins . This assembly of the pre-replicative complex (pre-RC) at origins during G1 phase is called ‘origin licensing’ FIG. The helicase is inactive in the pre-RC and is activated only in the S phase when origins ‘fire’ due to the activity of CDK/DDK kinases , . Once origins fire, DNA synthesis begins and the initiator proteins are degraded or exported out of the nucleus to prevent re-replication . The precise mechanisms of origin licensing and origin firing in two separate phases of the cell-cycle ensure that DNA replication occurs only once per cell-cycle.

  DNA synthesis

  The mechanism of DNA replication is greatly influenced by DNA structure. The complementary base pairing between the nitrogen bases A-T and G-C underlies the semi-conservative nature of DNA replication, which results in a duplicated genome with one parental strand and one newly synthesized strand. Each strand serves as a template for the DNA polymerase to catalyze the addition of the correct base during synthesis of a new complementary strand. As the strands are antiparallel with opposing polarity and since DNA polymerases can only synthesize DNA in the 5′ to 3′ direction, only one strand is continuously synthesized. This strand is called the leading strand. Synthesis of the other strand, called the lagging strand, is made possible through discontinuous synthesis of short fragments, called Okazaki fragments, in the 5′ to 3′ direction, which are later joined together.

  

  The replicating DNA: DNA replication proteins at the replication fork. The helicase unwinds the duplex DNA and Single Strand Binding proteins (SSBs) coat and stabilize single stranded DNA formed by strand separation. Topoisomerase is seen ahead of the fork removing superhelical tension caused by strand separation. Note that the leading strand is synthesized continuously in the 5′ to 3′ direction, whereas the lagging strand is synthesized discontinuously as short fragments called Okazaki fragments. The Polymerase α-primase complex synthesizes short RNA primers that are extended up to 30-40 nucleotides. Thereafter polymerase ε and polymerase δ takes up the job of faster and efficient strand synthesis on lagging and leading strands respectively. Ligase seals the gap between Okazaki fragments.

  DNA synthesis begins in S phase as the replicative helicase unwinds and separates the two strands of the DNA double helix . As the helicase unwinds DNA, DNA polymerase synthesizes DNA utilizing the exposed single stranded DNA as a template. DNA polymerases ‘read’ the template strand and add the correct complimentary base. Energy for polymerization comes from release of a pyrophosphate from a free deoxyribonucleotide triphosphate (dNTP), creating a 5′monophosphate that could be covalently linked to the 3′ hydroxyl group of another nucleotide. However, DNA polymerases cannot synthesize DNA de novo and require a preexisting primer with a free hydroxyl group to add nucleotides and extend the chain. A specialized RNA polymerase called primase synthesizes short RNA sequences about 10 nucleotides long which serve as primers. A single primer aids DNA replication on the leading strand and multiple primers initiate okazaki fragment synthesis on the lagging strand. In Eukaryotes, the primase is part of the DNA polymerase α (reviewed in ). The replicative helicase and primase functionally co-operate and stimulate each other’s activity .

  After DNA polymerase α has synthesized a short, 30-40 nucleotide stretch of DNA, further DNA synthesis is handed over to polymerase ε and polymerase δ which have a higher processivity than polymerase α. The higher processivity or the ability of the polymerases to stay associated with DNA for upto 10kb without falling off is due to their association with a sliding clamp called PCNA. The polymerase switching enables DNA synthesis with high fidelity as polymerase ε and polymerase δ have a 3′ – 5′ exonuclease activity which enables proof reading and removal of any incorrect bases that is incorporated (reviewed in ). At the replication fork, there is a division of labor between the polymerases where polymerase ε carries out leading strand synthesis and polymerase δ is involved in the synthesis of the lagging strand , 12)

  Okazaki fragment maturation and replication termination

  The Okazaki fragments which are about 100-200bp in eukaryotes are ligated together in a process known as Okazaki fragment maturation to complete DNA synthesis. Polymerase δ, as it runs into the adjacent Okazaki fragment ahead of polymerization removes 2 to 3 nucleotides of the RNA primer thereby generating a short flap that is processed by Fen1 . This leaves a nick that is sealed by DNA ligase1 . Although there are well-defined replications termination sequences called Ter sites in prokaryotes, in eukaryotes, termination typically occurs by the collision of two replication forks.

  DNA Replication: a mechanobiology perspective

  DNA replication begins with the unwrapping and unwinding of the highly compacted chromatin structure. The two strands of the double helix must also be separated before the replication machinery can access and copy each strand. Specialized ATPase motor proteins called helicases catalyze DNA unwinding by translocating along the DNA substrate and separating the base pairs .

  

  Model showing ATR-mediated checkpoint in response to mechanical stress created by DNA replication at the nuclear envelope: a) Nucleus showing DNA (dark blue strands) with regions tethered to the nuclear envelope b) Mechanical stress (red bar) created at the nuclear envelope by replicating DNA (red strands) c) Recruitment of ATR to the nuclear envelope transiently detaches DNA from the nuclear envelope allowing for completion of DNA replication. d) Nucleus showing newly replicated DNA (Adapted from Kumar et al, ATR mediates a checkpoint at the nuclear envelope in response to mechanical stress, Cell, 2014)

  As DNA unwinding and DNA synthesis progresses, the DNA ahead of the replication fork becomes overwound or positively supercoiled. This creates superhelical tension which is usually resolved by enzymes known as topoisomerases. However, the super helical tension is higher in longer chromosomes and in regions of the chromatin tethered to the nuclear envelope (reviewed in ). It is now evident that the torsional stress from the replication forks impinge on the nuclear envelope in the form of mechanical signals that recruit ATR, a DNA damage checkpoint protein, independent of its role in DNA repair . ATR may then enable transient detachment of chromatin from the nuclear envelope, thus allowing for the completion of replication .  ATR is also recruited during prophase to resolve the topological stress arising from chromatin condensation and is required for coordinating DNA replication and chromatin condensation.

  Aside from mechanical forces generated within the cell as a result of DNA replication itself, DNA replication may also be affected by external forces acting on the cell. It is well-known that external forces transduce to the nucleus via cytoskeletal links and affects gene regulation as well as organization of chromosomes . Therefore understanding DNA replication requires a mechanobiology perspective, incorporating the physical challenges of DNA packaging and unwinding as well as mechanical forces that influence DNA replication.

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  More Questions FAQ

  How is DNA packed inside the nucleus?Sruthi Jagannathan2018-01-17T12:47:16+08:30

  A series of processes must take place that enable the cell to package DNA within the confines of the nucleus whilst retaining its ability to transcribe and duplicate the entire DNA sequence and maintain its integrity. This is achieved through an elaborate process of DNA condensation that sees DNA packaged into 46 chromosomes (or 23 chromosome pairs) in humans.

  How is translation terminated?Sruthi Jagannathan2017-12-19T16:53:50+08:30

  The next step in the process of translation is termination. In this step an mRNA stop codon indicates that no additional amino acids are to be added to the growing protein.

  How are ribosomes recycled during translation?Sruthi Jagannathan2017-12-19T16:50:22+08:30

  The final step in translation is ribosome recycling, which sees the ribosome split into its smaller subunit parts and prepare for another round of translation. In eukaryotes this means the 80S ribosome splits into its 40S and 60S subunits.

  What happens during the elongation stage of translation?Sruthi Jagannathan2017-12-19T16:46:46+08:30

  Elongation occurs over several well-defined steps, beginning with the recognition of the mRNA codons by their corresponding aminoacyl-tRNA. Association with the mRNA occurs via the ribosomal A site and is influenced by various elongation factors.

  How is translation initiated?Sruthi Jagannathan2017-12-19T16:43:25+08:30

  The first step in translation is known as initiation. Here, the large (60S) and small (40S) ribosomal units are assembled into a fully functional 80S ribosome. This is positioned at the start codon (AUG) of the mRNA strand to be translated.

  What is translation?Sruthi Jagannathan2017-12-19T16:02:08+08:30

  Translation is a process that involves the synthesis of an amino acid chain from an mRNA blueprint. These polypeptide chains fold into functional proteins.

  What is euchromatin and heterochromatin?Sruthi Jagannathan2017-12-19T15:44:10+08:30

  Traditionally, chromatin is classified as either euchromatin or heterochromatin, depending on its level of compaction. Euchromatin has a less compact structure, and is often described as a 11 nm fiber that has the appearance of ‘beads on a string’ where the beads represent nucleosomes and the string represents DNA. In contrast, heterochromatin is more compact, and is often reported as being composed of a nucleosome array condensed into a 30 nm fiber.

  What are transcription factories?Sruthi Jagannathan2017-12-19T15:21:33+08:30

  Despite 20,000 genes being present in each haploid nucleus, the number of transcription foci is limited to around 2000. These transcription foci, also known as transcriptional factories are distinct submicron nuclear regions that are associated with nascent RNA production and are enriched in RNA polymerase II (RNA pol II) complexes.

  What are telomeres?Sruthi Jagannathan2017-12-19T15:14:11+08:30

  Telomeres are short nucleotide sequences found at the end of linear chromosomes which protect the genetic information. In vertebrates, telomeres have the hexameric sequence TTAGGG.

  What are the various models describing the structural organization of chromosome territories?Sruthi Jagannathan2017-12-19T14:32:09+08:30

  With the development of high-throughput biochemical techniques, such as 3C (‘chromosome conformation capture’) and 4C (‘chromosome conformation capture-on-chip’ and ‘circular chromosome conformation capture’), numerous spatial interactions between neighbouring chromatin territories have been described. Together, these observations and physical simulations have led to the proposal of various models that aim to define the structural organization of chromosome territories.

  How is transcription regulated in stem cells?Sruthi Jagannathan2017-12-19T13:59:25+08:30

  Embryonic stem cells are pluripotent in early organism development, but gradually undergo lineage restriction and transform into the stem cells with limited differentiation capacities (e.g., hematopoietic stem cells, neural stem cells).

  How is the nucleus maintained in a prestressed state?Sruthi Jagannathan2017-12-19T12:40:18+08:30

  As an integral part of cellular behavior, cells are sensitive to matrix rigidity, local geometry and stress or strain applied by external factors. In recent years, it has been established that an extensive network of protein assembly couples the cytoskeleton to the nucleus and that condensation forces of the chromatin balance cytoskeletal forces resulting in a prestressed nuclear organization.

  What are the stages in DNA replication?Sruthi Jagannathan2017-12-18T16:17:47+08:30

  Cells must replicate their DNA before they can divide. This ensures that each daughter cell gets a copy of the genome, and therefore, successful inheritance of genetic traits. DNA replication is an essential process and the basic mechanism is conserved in all organisms.

  How does chromatin remodeling regulate gene transcription?Sruthi Jagannathan2017-12-18T15:53:34+08:30

  While chromosome territory dynamics is believed to regulate gene expression through the redistribution of genes and the subsequent co-localization of these genes with transcription machinery, changes are also commonly made to the chromosome structure at a ‘local’ level. Although these changes do not necessarily involve the redistribution of genes, they do have a significant influence on gene regulation.

  How do chromosome territory dynamics regulate gene expression?Sruthi Jagannathan2017-12-18T15:22:28+08:30

  The spatial organization of chromatin within the 3-dimensional space of a chromosome territory enables the co-localization of co-transcribed genes and their transcriptional foci. Many gene positioning studies have shown that individual genes often loop out of their chromosomal territory to co-localize with transcription factories.

  What is the chromatin polymer model of chromosome territory organization?steve2018-01-19T15:08:09+08:30

  The chromatin polymer models assume a broad range of chromatin loop sizes and predict the observed distances between genomic loci and chromosome territories, as well as the probabilities of contacts being formed between given loci. These models apply physics-based approaches that highlight the importance of entropy for understanding nuclear organization…

  What is the Fraser and Bickmore model of chromosome territory organization?steve2018-01-19T15:06:27+08:30

  The Fraser and Bickmore model emphasizes the functional importance of giant chromatin loops, which originate from chromosome territories and expand across the nuclear space in order to share transcription factories. In this case, both cis- and trans- loops of decondensed chromatin can be co-expressed and co-regulated by the same transcription factory…

  What is the interchromatin network (ICN) model of chromosome territory organization?steve2018-01-19T15:12:33+08:30

  The interchromatin network (ICN) model of chromosome territory organization predicts that intermingling chromatin fibers/loops can make both cis- (within the same chromosome) and trans- (between different chromosomes) contacts. This intermingling is uniform and makes distinction between the chromosome territory and interchromatin compartment functionally meaningless…

  What models describe chromosome territories?steve2018-01-19T15:13:49+08:30

  With the development of high-throughput biochemical techniques, such as 3C (‘chromosome conformation capture’) and 4C (‘chromosome conformation capture-on-chip’ and ‘circular chromosome conformation capture’), numerous spatial interactions between neighbouring chromatin territories have been described. These descriptions have been supplemented with the construction of spatial proximity maps for the entire genome (e.g., for a human lymphoblastoid cell line). Together, these observations and physical simulations have led to the proposal of various models that aim to define the structural organization of chromosome territories…

  What are chromosome territories?steve2017-12-19T14:06:59+08:30

  During interphase, each chromosome occupies a spatially limited, roughly elliptical domain which is known as a chromosome territory (CT). Each chromosome territory is comprised of higher order chromatin units of ~1 Mb each. These units are likely built up from smaller loop domains.

  What are nucleosomes?steve2017-12-19T14:52:22+08:30

  In order to fit DNA into the nucleus, it must be packaged into a highly compacted structure known as chromatin. In the first step of this process DNA is condensed into a 11 nm fiber that represents an approximate 6-fold level of compaction. This is achieved through nucleosome assembly.

  How is DNA packed inside the nucleus?steve2017-12-19T15:34:48+08:30

  A series of processes must take place that enable the cell to package DNA within the confines of the nucleus whilst retaining its ability to transcribe and duplicate the entire DNA sequence and maintain its integrity. This is achieved through an elaborate process of DNA condensation that sees DNA packaged into 46 chromosomes (or 23 chromosome pairs) in humans.

  What constitutes the genome?steve2018-01-19T15:32:19+08:30

  The human genome contains over 3 billion base pairs or nucleotides. These nucleotides, which are arranged in a linear sequence along DNA (deoxyribonucleic acid), encode every protein and genetic trait in the human body…

  How does the cytoskeleton influence nuclear morphology and positioning?steve2018-01-19T16:12:40+08:30

  Work by Mazumder et al. ascertained the active involvement of cytoskeletal forces in determining nuclear morphology. Change in nuclear size upon perturbation of actomyosin and microtubules affirmed their roles in exerting tensile and compressive forces respectively on the nucleus, correlating with their functions in the cellular context , …

  How does the cytoskeleton couple the plasma membrane to the nucleus?steve2018-01-19T16:24:16+08:30

  Cytoskeletal filaments bridge the nucleus to the plasma membrane, which in turn is anchored at sub-cellular sites to extracellular substrates via a plethora of proteins that form focal adhesions (FAs). FAs are points of cross-talk between transmembrane integrin receptors and the cytoplasmic filaments and thus are key sites for both biochemical and mechanotransduction pathways…

  How is the organization and function of the genome regulated?steve2017-12-18T14:43:41+08:30

  Genome regulation encompasses all facets of gene expression, from the biochemical modifications of DNA, to the physical arrangement of chromosomes and the activity of the transcription machinery.The genome regulation programs that cells engage control which proteins are produced, and to what level. The programs are established during stem cell differentiation, and therefore dictate the specialized functions that the cell will carry out throughout its lifetime…

  What are intermediate chromatin structures?Andrew Wong2017-12-19T15:02:43+08:30

  Despite the extensive knowledge already gained on the structure of the 11 nm nucleosome fiber, as well as metaphase chromosomes, the intermediate chromatin structures commonly described are largely hypothetical and yet to be observed in vivo.Two popular models that were proposed based on in vitro data are the solenoid and zigzag.

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     How is DNA replication different during meiosis?

     During meiosis, 1 diploid cell undergoes 2 cycles of cell division but only 1 round of DNA replication. The result is 4 haploid daughter cells known as gametes. Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis.

     Is DNA replication the same in mitosis and meiosis?

     (i) Mitosis is preceded by one cycle of DNA replication and meiosis is preceded by two cycles of DNA replication. (ii) In sexually reproducing organisms, meiosis is the only type of cell division observed.

     Where is DNA replicated in meiosis?

     DNA replication takes place in S-phase of the Interphase before Meiosis I begins.

     How does DNA replication differ between mitosis and meiosis quizlet?

     Mitosis is different from Meiosis because meiosis creates cells with with half the number of chromosomes as the parent cell, while mitosis creates cells with the same number of Chromosomes as the parent cells.