How does crossing over look like

Since each chromatid is composed of a single deoxyribonucleic acid DNA duplex, the process of crossing-over involves the breakage and rejoining of DNA molecules. Although the precise molecular mechanisms have not been determined, it is generally agreed that the following events are necessary: 1 breaking nicking of one of the two strands of one or both nonsister DNA molecules; 2 heteroduplex hybrid DNA formation between single strands from the nonsister DNA molecules; 3 formation of a half chiasma, which is resolved by more single-strand breakages to result in either a reciprocal crossover, a noncrossover, or a nonreciprocal crossover conversion event.

Two molecular models of recombination which have gained credence are those of R. Holliday and of M. Meselson and C. Holliday's model postulates nicks in both chromatids at the initiation of crossing-over Fig. Meselson and Radding postulate single-strand cut in only one DNA strand.

Repair synthesis displaces this strand, which pairs with its complement on the other chromatid, thereby displacing and breaking the other strand of that DNA molecule. Following pairing and ligation of the two remaining broken ends, a half chiasma is formed. Other models have been postulated in which recombination is initiated by a double-stranded break in one chromatid. In all the above models, gene conversion can occur in the middle region of the molecules with or without outside marker crossing-over by mismatch repair of heteroduplex DNA.

Pachytene, the meiotic stage at which crossing-over is considered to occur, corresponds with the period of close pairing or synapsis of homologous chromosomes. Electron microscopy has revealed that proteinaceous structures, the synaptonemal complexes Fig.

A synaptonemal complex forms during zygotene by pairing of axial elements from two homologous chromosomes. It is present along the whole length of each pachytene bivalent and disappears at diplotene. Evidence from inhibitor studies and mutant stocks shows that the synaptonemal complex is necessary for meiotic crossing-over to occur.

However, in cases such as desynaptic mutants, some hybrids, and the female silkworm, complete pachytene synaptonemal complexes have been observed, but no crossing-over occurs, showing that the synaptonemal complex alone is not sufficient to cause crossing-over. In Drosophila melanogaster oocytes, the occurrence at pachytene of dense spherical bodies bridging the central region of the synaptonemal complex has been described.

These bodies coincided in number and position with expected crossover events, and therefore were named recombination nodules. A variety of oval and bar-shaped recombination nodules Fig. In many cases their number correlates with crossover frequency. It has been suggested that recombination nodules are prerequisites for crossing-over. If this is so, the recombination nodule may represent a complex of enzymes involved in the early events of recombination nicking, strand separation, repair synthesis.

DNA repair synthesis has been observed during pachytene in lily microsporocytes, and has been shown to be reduced in an achiasmatic mutant. Prophase I of lilies is characterized by the presence of several proteins which could have a role in crossing-over, for example, DNA binding protein, endonucleases, ligases, and kinase.

These links ensure that homologs are properly segregated during meiosis I. As you might expect, each chromosome usually undergoes at least one crossover. Crossovers can occur within genes or in noncoding regions outside of genes.

The location on a chromosome greatly affects recombination events. Identical sequences will have different levels of recombination depending on whether they are near telomeres, near centromeres, or in the middle of a chromosome arm. Also, crossovers inhibit one another. This prevents crossovers from occurring too close to each other on a chromosome.

Surprisingly, even sex can influence crossovers. Recombination frequencies are different in males and females. As you can see, many factors affect crossovers. During the G2 stage of the cell cycle, cells contain 4 copies of each gene. How does the cell deal with these excess in copy number? Hello Esther, It sounds like you are curious about cellular mechanisms that control gene expression during the G2 phase of the cell cycle, since twice as many chromosomes would mean twice the number of genes.

How do cells control the production of gene products? Cells use gene regulation mechanisms to control transcription precisely. Transcription of a gene by RNA polymerase can be regulated using several mechanisms, including specific factors that can alter the specificity of RNA polymerase to a promoter and repressors that impede RNA polymerase by binding to DNA sequences near promoter regions.

Additionally, transcription can be regulated by transcription factors that position RNA polymerase at protein-coding sequences, activators that enhance interactions between RNA polymerase and promoters, and enhancers that increase transcription.

The timing of transcription depends largely on the gene. RNA transcription is associated with a transcription bubble, which consists of an unwound template DNA that is accessible to RNA polymerases and transcription factors. Are you also curious about how RNA transcription is regulated when DNA is replicated during S-phase and when chromosomes are segregated during mitosis?

Transcription generally does not happen at the same time as DNA replication. As cells enter the mitotic phase of the cell cycle, a dramatic reorganization of the DNA into highly condensed chromosomes occurs, and gene transcription is silenced on a global scale. Hello Victor, We believe your question is asking about how genes are translated into proteins.

This is one of the most essential ideas in molecular biology. To state it more simply, the central dogma describes how a gene is expressed and made into a protein that can function in the cell. The basic process can be described using the following general steps: mRNA is transcribed from DNA in the nucleus, and then the mRNA is translated into protein in the cytoplasm.

The process of translation includes three steps: initiation, elongation, and termination. And I will draw this overlapping although they could have. Shorter one from the mother. And once again, each of these, this is a homologous pair, that's a homologous pair over there. Now, the DNA has been replicated so in each of the chromosomes in a homologous pair, you have two sister chromatids.

And so, in this entire homologous pair, you have four chromatids. And so, this is sometimes called a tetrad. So let me just give ourselves some terminology. So this right over here is called a tetrad or often called a tetrad. Now, the reason why I drew this overlapping is when we are in prophase I in meiosis I. Let me label this.

This is prophase I. You can get some genetic recombination, some homologous recombination. Once again, this is homologous pair. One chromosome from the father that I've gotten from the father. The species or the cell got it from its father's cell and one from the mother. And they're homologous. They might contain different base pairs, different actual DNA, but they code for the same genes. Over simplification, but in a similar place on each of these it might code for eye color or I don't know, personality.

Nothing is that simple in how tall you get and it's not that simple in DNA but just to give you an idea of how it is. And the reason why I overlapped them like this is to show how the recombination can occur. So actually, let me zoom in. So this is the one from the father. Once again, it's on the condensed form. This is one chromosome made up of two sister chromatids right over here.

And I drew the centromere, not to be confused with centrosomes. That's where they are, those sister chromatids are attached. And then, I will draw the homologous chromosome from the mother. So the homologous chromosome from the mother just like that. Homologous chromosome from the mother. And the recombination can occur at a point right over here. So after you're done with the recombination, this side might look something more like this.

So let me draw it like this. So, they essentially break up and they swap those little sections. There's one way to think about it. So this one, we'll now have a little piece from the mother. It might code for similar genes. But now it contains the mother's genetic information. And then this one over here will now have the piece. And you could say even homologous piece from the father. Let me do these two centromeres. And this is really interesting. All the time, there couldn't be recombination and often times it can lead to kind of non-optimal things, nonsense code and DNA.

It might lead to a nonfunctional organism. But this happens fairly common in the meiosis and it's a way, once again, to get more variation. We've talked about sexual reproduction before. And sexual reproduction introduces variation into a population.

And this, obviously, when different sperms find different eggs that introduces variation. But then, even amongst homologous pairs you can actually have exchange between this chromosome. And that's interesting because as we mentioned, each of these chromosomes, they code for a bunch of different genes.

And a gene is kinda looking code for a specific or a set of proteins. So this right over here, and this is what I'm about to say is gonna be huge over simplification. Maybe right over here you coded for eye color or it was related to, or it helps code for eye color.

And then you got that from your dad. And here, it helped code for eye color. And you got that from your mom. Your mom might have trended you towards a lighter eye color and your dad might have trended you towards a darker eye color. But now, the one from your mom is on this chromosome, this gene, and then the one or they've both the same gene. They're just different allele.

They're coding for different variance of that gene.