Information

7.2: Meiosis - Biology

7.2: Meiosis - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Sexual reproduction requires fertilization, a union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. The number of sets of chromosomes in a cell is called its ploidy level. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. If the reproductive cycle is to continue, the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division, known as meiosis, that reduces the number of chromosome sets.

Most animals and plants are diploid, containing two sets of chromosomes; in each somatic cell (the nonreproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to as homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found only within gametes. Gametes fuse with another haploid gamete to produce a diploid cell.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, mitosis is part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei contain the same number of chromosome sets—diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve the reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the stages are designated with a “I” or “II.” Thus, meiosis Iis the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis I reduces the number of chromosome sets from two to one. The genetic information is also mixed during this division to create unique recombinant chromosomes. Meiosis II, in which the second round of meiotic division takes place in a way that is similar to mitosis, includes prophase II, prometaphase II, and so on.

Interphase

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase is the first phase of interphase and is focused on cell growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G2 phase, the cell undergoes the final preparations for meiosis.

During DNA duplication of the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart during meiosis II. In an animal cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.

Meiosis I

Early in prophase I, the chromosomes can be seen clearly microscopically. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over. This process is revealed visually after the exchange as chiasmata (singular = chiasma) (Figure 7.2.1).

As prophase I progresses, the close association between homologous chromosomes begins to break down, and the chromosomes continue to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata (Figure 7.2.1) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete, it will carry some DNA from one parent of the individual and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes that did not exist before the crossover.

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. The microtubules assembled from centrosomes at opposite poles of the cell grow toward the middle of the cell. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome attached at one pole and the other homologous chromosome attached to the other pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the cell is random.

This randomness, called independent assortment, is the physical basis for the generation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. In metaphase I, these pairs line up at the midway point between the two poles of the cell. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations depends on the number of chromosomes making up a set. There are two possibilities for orientation (for each tetrad); thus, the possible number of alignments equals 2n where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (223) possibilities. This number does not include the variability previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 7.2.2).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes are recombined by crossover events occurring on each homologous pair during prophase I; in addition, the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

In anaphase I, the spindle fibers pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. It is the chiasma connections that are broken in anaphase I as the fibers attached to the fused kinetochores pull the homologous chromosomes apart (Figure 7.2.3).

In telophase I, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I.

Cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei in other organisms. In nearly all species, cytokinesis separates the cell contents by either a cleavage furrow (in animals and some fungi), or a cell plate that will ultimately lead to formation of cell walls that separate the two daughter cells (in plants). At each pole, there is just one member of each pair of the homologous chromosomes, so only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though there are duplicate copies of the set because each homolog still consists of two sister chromatids that are still attached to each other. However, although the sister chromatids were once duplicates of the same chromosome, they are no longer identical at this stage because of crossovers.

CONCEPT IN ACTION

Review the process of meiosis, observing how chromosomes align and migrate, at this site.

Meiosis II

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. In some species, cells enter a brief interphase, or interkinesis, that lacks an S phase, before entering meiosis II. Chromosomes are not duplicated during interkinesis. 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.

In prophase II, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. In 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. In metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell. In anaphase II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

In telophase II, the chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this point, the nuclei in the newly produced cells 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 recombination of maternal and paternal segments of chromosomes—with their sets of genes—that occurs during crossover.

Comparing Meiosis and Mitosis

Mitosis and meiosis, which are both forms of division of the nucleus in eukaryotic cells, share some similarities, but also exhibit distinct differences that lead to their very different outcomes. Mitosis is a single nuclear division that results in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: one in the case of haploid cells, and two in the case of diploid cells. On the other hand, meiosis is two nuclear divisions that result in four nuclei, usually partitioned into four new cells. The nuclei resulting from meiosis are never genetically identical, and they contain one chromosome set only—this is half the number of the original cell, which was diploid (Figure 7.2.4).

The differences in the outcomes of meiosis and mitosis occur because of differences in the behavior of the chromosomes during each process. Most of these differences in the processes occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together, experience chiasmata and crossover between sister chromatids, and line up along the metaphase plate in tetrads with spindle fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I, never in mitosis.

Homologous chromosomes move to opposite poles during meiosis I so the number of sets of chromosomes in each nucleus-to-be is reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level in mitosis.

Meiosis II is much more analogous to a mitotic division. In this case, duplicated chromosomes (only one set of them) line up at the center of the cell with divided kinetochores attached to spindle fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid is pulled to one pole and the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossovers, the two products of each meiosis II division would be identical as in mitosis; instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

Cells produced by mitosis will function in different parts of the body as a part of growth or replacing dead or damaged cells. They may even be involved in asexual reproduction in some organisms. Cells produced by meiosis in a diploid-dominant organism such as an animal will only participate in sexual reproduction.

CONCEPT IN ACTION

For an animation comparing mitosis and meiosis, go to this website.

Section Summary

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. Meiosis is a series of events that arrange and separate chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four haploid daughter cells, each with half the number of chromosomes as the parent cell. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment at metaphase I. The cells that are produced by meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions are two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set rather than the two sets the parent cell had. The main differences between the processes occur in the first division of meiosis. The homologous chromosomes separate into different nuclei during meiosis I causing a reduction of ploidy level. The second division of meiosis is much more similar to a mitotic division.

Multiple Choice

Meiosis produces ________ daughter cells.

A. two haploid
B. two diploid
C. four haploid
D. four diploid

C

At which stage of meiosis are sister chromatids separated from each other?

A. prophase I
B. prophase II
C. anaphase I
D. anaphase II

D

The part of meiosis that is similar to mitosis is ________.

A. meiosis I
B. anaphase I
C. meiosis II
D. interkinesis

C

If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that same organism?

A. 8
B. 16
C. 32
D. 64

B

Free Response

Explain how the random alignment of homologous chromosomes during metaphase I contributes to variation in gametes produced by meiosis.

Random alignment leads to new combinations of traits. The chromosomes that were originally inherited by the gamete-producing individual came equally from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell to form a tetrad. The orientation of each tetrad is random. There is an equal chance that the maternally derived chromosomes will be facing either pole. The same is true of the paternally derived chromosomes. The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled apart in anaphase I, any combination of maternal and paternal chromosomes will move toward each pole. The gametes formed from these two groups of chromosomes will have a mixture of traits from the individual’s parents. Each gamete is unique.

In what ways is meiosis II similar to and different from mitosis of a diploid cell?

The two divisions are similar in that the chromosomes line up along the metaphase plate individually, meaning unpaired with other chromosomes (as in meiosis I). In addition, each chromosome consists of two sister chromatids that will be pulled apart. The two divisions are different because in meiosis II there are half the number of chromosomes that are present in a diploid cell of the same species undergoing mitosis. This is because meiosis I reduced the number of chromosomes to a haploid state.

Glossary

chiasmata
(singular = chiasma) the structure that forms at the crossover points after genetic material is exchanged
crossing over
(also, recombination) the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents of the organism forming reproductive cells
fertilization
the union of two haploid cells typically from two individual organisms
interkinesis
a period of rest that may occur between meiosis I and meiosis II; there is no replication of DNA during interkinesis
meiosis I
the first round of meiotic cell division; referred to as reduction division because the resulting cells are haploid
meiosis II
the second round of meiotic cell division following meiosis I; sister chromatids are separated from each other, and the result is four unique haploid cells
recombinant
describing something composed of genetic material from two sources, such as a chromosome with both maternal and paternal segments of DNA
reduction division
a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division
somatic cell
all the cells of a multicellular organism except the gamete-forming cells
synapsis
the formation of a close association between homologous chromosomes during prophase I
tetrad
two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I

7.2: Meiosis - Biology

Click on one of the following links to jump to a specific subtopic on this page, or scroll the text up and down to find what you need.

We said that meiosis is the two step division process by which sperm and eggs are produced. But there are several concepts that need to be addressed before we look at that process.

We also said that humans have 46 chromosomes in each somatic cell. However, since humans have two of every type of chromosome (not counting gender differences), that means that we really have two sets of 23 chromosomes. One set came from your mother and one set came from your father. Notice here that each parent is contributing only 23 chromosomes, which is one half of the total in somatic cells. Therefore, meiosis has to produce specialized reproductive cells called gametes in general, sperm and eggs specifically, which have one of every type of chromosome. After fertilization, then the number of chromosomes in the developing zygote is returned to 46.

Organisms or cells with two of every type of chromosomes are called diploids. Organisms or cells with one of every type of chromosomes are called haploids. Human somatic cells are diploid while the gametes are haploid. Meiosis then is a process by which haploid cells are produced.

There is an additional concept which uses the letter N to represent the haploid number of chromosomes. Therefore, gametes are 1N while somatic cells are 2N.

Also, the letter C is used to represent a haploid amount of DNA in a cell. Remember that unreplicated chromosomes have only one piece of DNA each while replicated chromosomes have two pieces of DNA each (refer to previous key word list). Therefore, a 1N cell can be 1C or 2C and a 2N cell can be 2C or 4C depending on the replication state of their chromosomes.

A cell ready to undergo meiosis (2N, 2C), enters in to one last Interphase where its chromosomes are replicated (2N, 4C). If this were mitosis, it would remain in this state until Anaphase when the chromatids separate forming two daughter cells (2N, 2C each). However, in the first meiotic division (called meiosis I), during Prophase while the chromosomes are condensing, each type of chromosome pairs up lengthwise with the other chromosome of that type ultimately forming 23 pairs of chromosomes. This pairing process is called synapsis. Chromosomes of the same type are called homologous chromosomes and each member of a pair is referred to as a homolog or homologue. At Anaphase of meiosis I, rather than chromatids separating, the homologs separate with one of each pair going to a different daughter cell (1N, 2C). In this way, the chromosome number is reduced in half during meiosis I and two haploid cells are produced. Because of this, meiosis I is called the Reductional Division.

After meiosis I, there is no additional Interphase. Sometimes the cells produced in meiosis I will go in to a prolonged resting state which is referred to as interkinesis. In meiosis II, each of the two daughter cells produced in meiosis I will divide essentially as occurs in mitosis. It is in meiosis II that the chromatids separate forming a total of four daughter cells (1N, 1C) for every cell entering in to meiosis. Meiosis II is called the Equational Division.

In males, four sperm are produced for each cell undergoing meiosis. But in females, certain special events occur through which only one mature egg will be produced per starting cell. Meiosis I takes place as above. However, of the two potential cells to be produced, one of them keeps the bulk of the cytoplasm to itself leaving the other nucleus without cytoplasm. This nucleus which is called a primary polar body will degenerate and the chromosomes it contains will be lost. The same thing happens in meiosis II forming what is called a secondary polar body which is also lost. So, for each cell starting meiosis in females, only one mature egg is produced carrying most of the cytoplasm of the original cell. This is necessary since it is the egg which provides the cytoplasm from which the fetus will develop. There is not enough in the starting cell to produce four eggs.


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

Figure 1. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.


7.2 Sex is a Problem

Between the cost of meiosis, the increased risk of disease transmission, and the cash required to buy dinner and a movie, sex is expensive! So why does it exist and why is it the major method of reproduction for multicellular organisms? The short answer to this question is, “nobody knows.” But like most short answers in science, that is not the interesting answer. Of the many hypotheses, a few consistently arise to explain the evolution and persistence of sex: 1) sex speeds up evolution 2) sex leads to fewer mutations and 3) sex is an evolutionary race between parasites and hosts.

Figure 7.2 Various organisms thrive without sex. A few examples include a) the bdelloid rotifers b) whiptail lizards c) dandelions and d) fungus-farming ants.

Does sex speed up evolution?

Several biologists have suggested that the purpose of sex is to speed up evolution. Between meiosis and exchanging gametes with another individual (or outcrossing), the process of sex produces a wide variety of offspring this variety could potentially speed up the process of evolution.

One key prediction of this hypothesis is that species that evolve quickly have an advantage over species that evolve slowly. Unfortunately, this hypothesis is difficult to test, in that it makes a species-level claim about a mechanism (natural selection) that occurs at the individual level. An individual’s fiercest competitors are those that are closely related and vie for the same resources. So imagine an asexually reproducing population that has a low-frequency variant that reproduces sexually. The sexually reproducing individuals would have to have some sort of advantage over their asexual counterparts. Merely evolving at a more rapid pace does not necessarily give an immediate advantage to those individuals.

Additionally, while it may be true that sexually reproducing populations can change more quickly than asexuals, it does not necessarily follow that this is a good thing. Plenty of successful sexual organisms have changed little across long spans of time. For example, the coelacanth fish that exists today seems nearly identical to fossilized coelacanths that existed millions of years ago (Figure 7.3 and 7.4).

Figure 7.3 Image of a fossilized coelacanth from millions of years ago Figure 7.4 Image of a coelacanth swimming today

Perhaps the largest problem with the idea that sex exists “to speed up evolution” is the premise that evolution itself is the goal. Change for the sake of change is not the point of evolution. In the case of natural selection, populations change in response to selective pressures. As part of this change, there are winners, to be sure, but there are also many losers.


07 Nucleic acids AHL

DNA structure

  • Part of DNA supercoiling are structures called Nucleosomes.
  • DNA structure gives a clue to the mechanism of DNA replication.
  • Non-coding regions of DNA have other important functions, limited to regulators of gene expression, introns, telomeres and genes for tRNAs.

DNA replication (in prokaryotes only)

  • DNA polymerase enzymes can only add nucleotides to the 3&rsquo end of a primer.
  • Continuous DNA replication occurs on the leading strand and discontinuous on the lagging strand.
  • A complex group of enzymes do DNA replication including helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III.
  • DNA replication makes a second chromatid in each chromosome in interphase before meiosis.
  • Crossing over exchanges pieces of DNA between non-sister homologous chromatids and forms new combinations of alleles on the chromosomes formed in meiosis.
  • See that Rosalind Franklin&rsquos and Maurice Wilkins&rsquo X-ray diffraction work gave evidence for helix and two strands in DNA structure.
  • Awareness that the Sanger method of base sequencing uses nucleotides containing dideoxyribonucleic acid (DNA with deoxyribose missing 2 oxygen molecules) to stop DNA replication at a specific base which allows sequencing using fluorescent markers and computers. (Sanger chain termination. Video here)
  • Awareness that in DNA profiling Tandem repeats are used as these vary greatly from person to person.
  • Ability to analyse of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.(This is a nice graphic)
  • Analyse of molecular visualizations of the association between protein and DNA in a nucleosome

7.2 Transcription

  • The direction of Transcription is in a 5&rsquo to 3&rsquo direction as RNA polymerase adds the 5´ end (phosphate) of the free RNA nucleotide to the 3´ end of the growing mRNA molecule.
  • Transcription is partly regulated by Nucleosomes in eukaryotes.
  • Eukaryotes modify mRNA after transcription.
  • Splicing of mRNA increases the number of different proteins an organism can produce.
  • Gene expression is regulated by proteins that bind to specific base sequences in DNA. - eg. methylation
  • Gene expression is affected by the environment of a cell and of an organism.

Skills

  • Awareness that the promoter region is an example of non-coding DNA.
  • The skill to analyse changes in the DNA methylation patterns in connection with gene expression

7.3 Translation

Three stages of translation

  • Initiation is the assembly of the components (large and small ribosome subunits, mRNA and tRNA molecules) that carry out the process.
  • Synthesis of the polypeptide involves a repeated cycle on a ribosome where tRNA binds to the A (aminoacyl), P (peptidyl) and E (exit) sites in turn. Polypeptide molecule is produced.
  • (examples of start and stop codons not needed)
  • Termination of translation is followed by disassembly of the components.

Ribosomes

  • Free ribosomes synthesize proteins for use primarily within the cell.
  • Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
  • Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.
  • The sequence and number of amino acids in the polypeptide is the primary structure.
  • The secondary structure is the formation of alpha helices and beta pleated sheets stabilised by hydrogen bonding.
  • The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups. (Polar and non-polar amino acids are relevant to the bonds formed between R groups.)
  • The quaternary structure exists in proteins with more than one polypeptide chain. and may involve the binding of a prosthetic group to form a conjugated protein.

Nucleic acids 7.1 HL

The basics of DNA structure and DNA replication were covered in the SL topic so this topic looks at some extra details including, the 3' and 5' ends of the two antiparallel strands and further details about the enzymes responsible for DNA replication.

Transcription 7.2 HL

This topic includes some of the more difficult details about the process of transcription of mRNA from DNA. The way that splicing of mRNA to remove introns can also make different mature mRNA molecules leads to an increase in the number of proteins.

Translation 7.3 HL

This topic includes some interesting and difficult details about the synthesis of proteins on ribosomes. While these aspects are difficult they do not need to understood in great detail.


Stages of Meiosis: Daughter Cells

The final result of meiosis is the production of four daughter cells. These cells have one half the number of chromosomes as the original cell. Only sex cells are produced by meiosis. Other cell types are produced by mitosis. When sex cells unite during fertilization, these haploid cells become a diploid cell. Diploid cells have the full complement of homologous chromosomes.


Watch the video: u0026 - Transcription (February 2023).