11.3
交叉
有糸分裂とは異なり、減数分裂は、半数体配偶子を形成する際に遺伝的多様性を生み出すことを目的とする。分裂する生殖細胞は、まず前期Iにこの過程を開始する。この段階では、S期に複製された各染色体は、セントロメアで結合した2本の姉妹染色分体（同一のコピー）から構成されている。
次に、相同染色体—一方は母方由…
ヒトは遺伝子的に異なる卵子細胞と 精子細胞を産生し、唯一の子孫を産生する それは、減数分裂過程の乗り換えの結果である。生殖器内では、核の二重前駆細胞乗り換えが 減数分裂過程初期の時期に行われ、それを第一期と呼んでいる。すでに全ての細胞の染色体は複写と 濃縮され卵型の形を作り出す。２つのセットとなったXは細胞に現れる。１つは母由来でもう１つは父由来である。重要な点は、それぞれのXの腕は同じ両親の 染色体のコピーであり、この複写であるペアは姉妹染色体である 母由来と父由来が同じバージョンの染色体は ペアとなり、繋がり始める ２つの間にたんぱく質の枠組みが出来る事を 対合複合体と言う。その結果、同じ母由来と同じ父由来の遺伝子が 一致する様に並び、相同染色体のペアが繋がる ここで非姉妹染色体でない部分は 切れてしまい 切れた部分は反対側の染色体へ 再結合する この乗り換え後には 対合複合体は消滅するが 相同染色体のペアは遺伝子伝達の過程で しっかりと繋がり、それ個別にはキアズマと呼ばび そのほとんどは減数分裂過程の第一分裂期である。それゆえ、乗り換えた先端の染色体は 両親の新たなユニークな組み合わせであり 遺伝子組み換えの例である。
View the full transcript and gain access to JoVE Core videos
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.