5.12 Sexual Reproduction, Meiosis, and Gametogenesis
Created by: CK-12/Adapted by Christine Miller
All in the Family
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex — sexual reproduction, that is.
Sexual Reproduction
Reproduction is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually. Sexual reproduction involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells — called gametes — that unite to form an offspring. Gametes are haploid (or 1N) cells. This means they contain one copy of each chromosome in the nucleus. Gametes are produced by a type of cell division called meiosis, which is described in detail below. The process in which two gametes unite is called fertilization. The fertilized cell that results is referred to as a zygote. A zygote is a diploid (or 2N) cell, which means it contains two copies of each chromosome. Thus, it has twice the number of chromosomes as a gamete.
Meiosis
The process that produces haploid gametes is called meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes separate, and four haploid cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.
As you can see in the meiosis diagram, two cell divisions occur during the overall process, producing a total of four haploid cells from one parent cell. The two cell divisions are called meiosis I and meiosis II. Meiosis I begins after DNA replicates during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from mitosis, the division of the nucleus that takes place during routine cell division of eukaryotic cells.
Meiosis I- Increasing genetic variation
The phases of Meiosis I are:
- Prophase I: The nuclear envelope begins to break down, and the chromosomes condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of mitosis and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.
- Metaphase I: Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment. The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.
- Anaphase I: Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.
- Telophase I and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.
Meiosis II- Halfing the DNA
The phases of Meiosis II are:
- Prophase II: The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.
- Metaphase II: Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.
- Anaphase II: Sister chromatids separate and move to opposite poles.
- Telophase II and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.
Sexual Reproduction and Genetic Variation
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring two individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps — and often a certain amount of luck — to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for genetic variation in offspring, which increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:
- When homologous chromosomes pair up during meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment. It results in gametes that have unique combinations of chromosomes. You can see how it happens in Figure 5.12.7.
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.
With all of this recombination of genes, there is a need for a new set of vocabulary. Remember, that sister chromatids are two identical pieces of DNA connected at a centromere. Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads. In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as tetrads.
All of these mechanisms — crossing over, independent assortment, and the random union of gametes — work together to result in an amazing range of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!
Meiosis (updated), Amoeba Sisters, 2017.
Gametogenesis
At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of fertilization. The development of haploid cells into gametes is called gametogenesis. It differs between males and females.
- A gamete produced by a male is called a sperm, and the process that produces a mature sperm is called spermatogenesis. During this process, a sperm cell grows a tail and gains the ability to “swim,” like the human sperm cell shown in Figure 5.12.8.
- A gamete produced by a female is called anegg or ovum, and the process that produces a mature egg is called oogenesis, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.
5.12 Summary
- In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with one copy of each of the 23 chromosomes, and the zygote is a diploid cell with two copies of each of the 23 chromosomes.
- Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).
- Meiosis is followed by gametogenesis, the process during which the haploid daughter cells change into mature gametes. Males produce gametes called sperm in a process known as spermatogenesis, and females produce gametes called eggs in the process known as oogenesis.
- Sexual reproduction produces genetically unique offspring. Crossing-over, independent alignment, and the random union of gametes work together to result in an amazing range of potential genetic variation.
5.12 Review Questions
-
- Explain how sexual reproduction happens at the cellular level.
- Summarize what happens during Meiosis.
- Compare and contrast gametogenesis in males and females.
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?
5.12 Explore More
Meiosis: Where the Sex Starts – Crash Course Biology #13, CrashCourse, 2012.
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.
Attributions
Figure 5.12.1
Family portrait by loly galina on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 5.12.2
Human Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0 (https://creativecommons.org/licenses/by-nc-sa/4.0/) license.
Figure 5.12.3
MajorEventsInMeiosis_variant_int by PatríciaR (internationalization) on Wikimedia Commons is used and adapted by Christine Miller. This image in the public domain. (Original image from NCBI; original vector version by Jakov.)
Figure 5.12.4
Meiosis 1/ Meiosis Stages by Ali Zifan on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 5.12.5
Meiosis 2/ Meiosis Stages by Ali Zifan on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 5.12.6
Crossover/ Figure 17 02 01 by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.
Figure 5.12.7
Independent_assortment by Mtian20 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 5.12.8
sperm fertilizing egg by AndreaLaurel on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
References
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https://www.youtube.com/watch?v=VzDMG7ke69g&feature=youtu.be
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https://www.youtube.com/watch?v=zrKdz93WlVk&feature=youtu.be
CrashCourse, (2012, April 23). Meiosis: Where the sex starts – Crash Course Biology #13. YouTube. https://www.youtube.com/watch?v=qCLmR9-YY7o&feature=youtu.be
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. In OpenStax, Biology (Section 17.2). http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.
By Christine Miller
DNA Replication: Overview
DNA replication is required for the growth or replication of an organism. You started as one single cell and are now made up of approximately 37 trillion cells! Each and every one of these cells contains the exact same copy of DNA, which originated from the first cell that was you. How did you get from one set of DNA, to 37 million sets, one for each cell? Through DNA replication.
Knowledge of DNA’s structure helped scientists understand DNA replication, the process by which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle. DNA must be copied so that each new daughter cell will have a complete set of chromosomes after cell division occurs.
DNA replication is referred to as "semi-conservative". What this means is when a strand of DNA is replicated, each of the two original strands acts as a template for a new complementary strand. When the replication process is complete, there are two identical sets of DNA, each containing one of the original strands of DNA, and one newly synthesized strand.
DNA replication involves a certain sequence of events. For each event, there is a specific enzyme which facilitates the process. There are four main enzymes that facilitate DNA replication: helicase, primase, DNA polymerase, and ligase.
DNA Replication: The Process
DNA replication begins when an enzyme called helicase unwinds, and unzips the DNA molecule. If you recall the structure of DNA, you may remember that it consists of two long strands of nucleotides held together by hydrogen bonds between complementary nitrogenous bases. This forms a ladder-like structure which is in a coiled shape. In order to start DNA replication, helicase needs to unwind the molecule and break apart the hydrogen bonds holding together complementary nitrogenous bases. This causes the two strands of DNA to separate.
Small molecules called single-stranded binding proteins (SSB) attach to the loose strands of DNA to keep them from re-forming the hydrogen bonds that helicase just broke apart.
Once the nitrogenous bases from the inside of the DNA molecule are exposed, the creation of a new, complementary strand can begin. DNA polymerase creates the new strand, but it needs some help in finding the correct place to begin, so primase lays down a short section of RNA primer (shown in green in Figure 5.4.3). Once this short section of primer is laid, DNA polymerase can bind to the DNA molecule and start connecting nucleotides in the correct order to match the sequence of nitrogenous bases on the template (original) strand.
If we think about the DNA molecule, we may remember that the two strands of DNA run antiparallel to one another. This means that in the sugar-phosphate backbone, one strand of the DNA has the sugar oriented in the "up" position, and the other strand has the phosphate oriented in the "up" position (see Figure 5.4.4). DNA polymerase is an enzyme which can only work in one direction on the DNA molecule. This means that one strand of DNA can be replicated in one long string, as DNA polymerase follows helicase as it unzips the DNA molecule. This strand is termed the "leading strand". The other strand, however, can only be replicated in small chunks since the DNA polymerase replicates in the opposite direction that helicase is unzipping. This strand is termed the "lagging strand". These small chunks of replicated DNA on the lagging strand are called Okazaki fragments.
Take a look at Figure 5.4.5 and find the Okazaki fragments, the leading strand and the lagging strand.
Once DNA polymerase has replicated the DNA, a third enzyme called ligase completes the final stage of DNA replication, which is repairing the sugar-phosphate backbone. This connects the gaps in the backbone between Okazaki fragments. Once this is complete, the DNA coils back into its classic double helix structure.
Semi-Conservative Replication
When DNA replication is complete, there are two identical sets of double stranded DNA, each with one strand from the original, template, DNA molecule, and one strand that was newly synthesized during the DNA replication process. Because each new set of DNA contains one old and one new strand, we describe DNA as being semi-conservative.
Watch this video for a great overview of DNA replication:
https://www.youtube.com/watch?v=Qqe4thU-os8
DNA Replication (Updated), Amoeba Sisters, 2019.
5.4 Summary
- DNA replication requires the action of three main enzymes each with their own specific role:
- Helicase unzips and unwinds the DNA molecule.
- DNA polymerase creates a new complementary strand of DNA on each of the originals halves that were separated by helicase. New nucleotides are added through complementary base pairing: A pairs with T, and C with G.
- Ligase repairs gaps in the sugar-phosphate backbone between Okazaki fragments.
- DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new complementary strand.
5.4 Review Questions
2. Why are Okazaki fragments formed?
- Because helicase only unzips DNA in one direction.
- Because DNA is in a double helix.
- Because DNA polymerase only replicates DNA in one direction.
- Because DNA replication is semi-conservative.
3. Drag and drop to label the diagram.
5.4 Explore More
https://www.youtube.com/watch?v=TNKWgcFPHqw
DNA replication - 3D, yourgenome, 2015.
Attributions
Figure 5.4.1
DNA_replication_split.svg by Madprime on Wikimedia Commons is used under a CC0 1.0
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 5.4.2
Helicase and single stranded binding proteins (1) by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.3
DNA polymerase and primase by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.4
DNA strands run antiparallel by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.5
Leading and lagging strand/ DNA Replication/ by yourgenome on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
References
Amoeba Sisters. (2019, June 28). DNA replication (Updated). YouTube. https://www.youtube.com/watch?v=Qqe4thU-os8&feature=youtu.be
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.24 DNA Replication [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-3-the-nucleus-and-dna-replication CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
yourgenome. (2015, June 26). DNA replication - 3D. YouTube. https://www.youtube.com/watch?v=TNKWgcFPHqw&feature=youtu.be