11.3
Crossing-over
Diferentemente da mitose, a meiose tem como objetivo promover a diversidade genética na formação de gametas haploides. As células germinativas em divi…
Humanos produzem óvulos geneticamente distintos e células espermáticas e, portanto, de descendência única como resultado do processo mitótico de crossing over. Nos órgãos sexuais, o cruzamento ocorre dentro dos núcleos de células precursoras diploides durante a primeira fase da Meiose I, chamado prófase I.Anteriormente, todos os cromossomas das células se replicavam e se condensavam em estruturas com forma de ovo. Dois conjuntos de X's são visíveis em uma célula.
Um de origem maternal e o outro paternal. É importante ressaltar que cada braço de um x, é uma cópia do mesmo cromossoma parental e esses pares duplicados são denominados cromatídeas irmãs. As versões maternal e paternal do mesmo cromossoma então começam a se emparelhar e a ficarem ligadas com uma estrutura proteica manifestada entre elas chamada de complexo Sinápio.
O resultado são pares conectados de cromossomas homólogos. Alinhados para que os mesmos genes maternos e paternos coincidam. Eles começam a se entrelaçar.
O material genético nos sítios onde as cromatídeas não-irmãs se cruzam, se quebra e os segmentos desconectados se reconectam a cromossomos opostos. Depois do crossing over o complexo sinápio se dissipa, mas os pares homólogos permanecem presos em pontos de transferência genética. Chamados individualmente de Chiasma, durante a maior parte da meiose I.Assim, o cruzamento termina com novas cromatídeas, combinações exclusivas de informações dos pais.
E como resultado, é um exemplo de recombinação genética.
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Q1: When does crossing over occur during meiosis?
Crossing over occurs during Prophase I, the first stage of Meiosis I. At this point, chromosomes have already replicated into X-shaped structures called sister chromatids. Homologous chromosomes—one maternal and one paternal—pair up and align alongside each other in a process called synapsis, facilitated by the synaptonemal complex protein framework.
Q2: What are sister chromatids and how do they relate to crossing over?
Sister chromatids are identical copies of the same chromosome joined at the center, created during chromosome replication before Prophase I. During crossing over, genetic material exchanges occur between non-sister chromatids—copies from different homologous chromosomes—not between sister chromatids. This exchange creates new combinations of alleles on each chromatid.
Q3: What is the synaptonemal complex and what does it do?
The synaptonemal complex is a protein framework that forms between paired homologous chromosomes during Prophase I. It holds maternal and paternal chromosome versions together in precise alignment, matching corresponding DNA positions. This structure facilitates the exchange of genetic material between non-sister chromatids, enabling homologous recombination and genetic diversity.
Q4: How does crossing over create genetic diversity?
Crossing over exchanges segments of DNA between non-sister chromatids of homologous chromosomes, creating new combinations of alleles. This process, called genetic recombination, produces chromatids with unique blends of maternal and paternal genetic information. The result is genetically distinct egg and sperm cells, ensuring unique offspring through meiosis and gamete formation.
Q5: What are chiasmata and when do they form?
Chiasmata are X-shaped structures that mark the physical points where crossing over occurred between homologous chromosomes. They form after the synaptonemal complex begins to dissolve but remain visible, holding the homologous chromosome pairs together until recombination is completed. Each chiasma represents one site of genetic transfer between non-sister chromatids.
Q6: Why is crossing over important for producing unique offspring?
Crossing over shuffles parental genetic information by exchanging DNA segments between homologous chromosomes, creating new allele combinations on each chromatid. This genetic recombination ensures that each egg and sperm cell produced is genetically distinct. Without crossing over, offspring would inherit unchanged chromosome sets from each parent, reducing genetic variation.
Q7: What happens to homologous chromosomes after crossing over ends?
After crossing over is complete, the synaptonemal complex dissolves, but homologous chromosome pairs remain connected at the chiasma sites throughout most of Meiosis I. These connection points hold the paired chromosomes together until they separate during the later stages of meiosis. The chromatids now carry recombined genetic material from both parental sources.