📚 Chromosomes | 染色体
Chromosomes are the packaged form of DNA in cells. They carry genetic information in the form of genes and are essential for accurate transmission of hereditary material during cell division. Understanding chromosome structure and behaviour is central to A-Level Biology, especially in topics on mitosis, meiosis, inheritance and genetic disorders.
染色体是细胞中 DNA 的包装形式。它们以基因的形式携带遗传信息，对于细胞分裂过程中遗传物质的准确传递至关重要。理解染色体的结构和行为是 A-Level 生物学的核心内容，尤其是在有丝分裂、减数分裂、遗传和遗传病等主题中。
1. What Are Chromosomes? | 什么是染色体？
In eukaryotic cells, chromosomes are highly organised structures made of DNA and proteins. They are located in the nucleus and become visible as distinct bodies when chromatin condenses during prophase of cell division. Each chromosome carries many genes arranged along its length.
在真核细胞中，染色体是由 DNA 和蛋白质组成的高度有序结构。它们位于细胞核内，当染色质在细胞分裂前期凝缩时，染色体就以清晰的个体显现出来。每条染色体沿其长度携带许多基因。
Before cell division, the genetic material exists as chromatin, a complex of DNA and histone proteins. Chromatin allows the long DNA molecules to fit into the nucleus and also helps regulate gene expression. Prokaryotic cells usually have a single circular chromosome, whereas eukaryotic cells have multiple linear chromosomes.
在细胞分裂前，遗传物质以染色质的形式存在，染色质是 DNA 与组蛋白的复合体。染色质使长链 DNA 分子能够装入细胞核，同时也有助于调控基因表达。原核细胞通常只有一条环状染色体，而真核细胞则有多条线状染色体。
2. Chromosome Structure: DNA and Histones | 染色体结构：DNA 与组蛋白
At the molecular level, DNA wraps around an octamer of histone proteins (two copies each of H2A, H2B, H3 and H4) to form a nucleosome, the basic unit of chromatin. About 146 base pairs of DNA are wound around each histone core, and linker DNA connects adjacent nucleosomes.
在分子水平上，DNA 缠绕在组蛋白八聚体（H2A、H2B、H3 和 H4 各两个拷贝）周围，形成核小体，即染色质的基本单位。约 146 个碱基对的 DNA 缠绕在每个组蛋白核心上，连接 DNA 将相邻的核小体连接起来。
Nucleosomes are further coiled and folded into a 30 nm chromatin fibre, and during mitosis this fibre is compacted even more to produce the metaphase chromosome. Euchromatin is less condensed and transcriptionally active, whereas heterochromatin remains tightly packed and largely inactive.
核小体进一步螺旋和折叠成 30 nm 染色质纤维，在有丝分裂期间，这种纤维被进一步压缩，形成中期染色体。常染色质凝缩程度较低，转录活跃；而异染色质保持高度紧密，通常不活跃。
3. Centromeres and Telomeres | 着丝粒与端粒
The centromere is a constricted region of a chromosome that holds sister chromatids together. It contains repeated DNA sequences and serves as the attachment site for kinetochore proteins, to which spindle microtubules bind during cell division.
着丝粒是染色体上一个缢缩的区域，它将姐妹染色单体连接在一起。着丝粒含有重复 DNA 序列，并作为动粒蛋白的附着位点，细胞分裂时纺锤体微管就结合在动粒上。
Based on centromere position, chromosomes are classified as metacentric, submetacentric, acrocentric or telocentric. This classification is used when arranging a karyotype.
根据着丝粒的位置，染色体可分为中着丝粒、亚中着丝粒、近端着丝粒和端着丝粒染色体。在排列核型时使用这一分类。
Telomeres are repetitive DNA sequences at the ends of linear chromosomes. In humans the repeat is TTAGGG. Telomeres protect chromosome ends from degradation and prevent fusion with other chromosomes, but they shorten with each round of DNA replication in somatic cells.
端粒是线性染色体末端的重复 DNA 序列。在人类中该重复序列为 TTAGGG。端粒保护染色体末端不被降解，并防止与其他染色体融合，但在体细胞中，端粒会随着每一轮 DNA 复制而缩短。
4. Homologous Chromosomes and Alleles | 同源染色体与等位基因
In a diploid cell, chromosomes occur in pairs called homologous chromosomes. One homologue is inherited from the mother and the other from the father. Homologous chromosomes have the same length, centromere position and gene loci, although they may carry different alleles.
在二倍体细胞中，染色体成对出现，称为同源染色体。一条同源染色体来自母方，另一条来自父方。同源染色体具有相同的长度、着丝粒位置和基因座，但可能携带不同的等位基因。
Alleles are alternative versions of a gene located at the same locus on homologous chromosomes. For example, a gene for seed shape may have a round allele and a wrinkled allele. This pairing is essential for Mendelian segregation during meiosis.
等位基因是位于同源染色体相同基因座上的基因的不同版本。例如，控制种子形状的基因可能有圆形等位基因和皱缩等位基因。这种配对对于减数分裂中孟德尔式分离至关重要。
5. Sister Chromatids and the Cell Cycle | 姐妹染色单体与细胞周期
After DNA replication in S phase, each chromosome consists of two identical sister chromatids joined at the centromere. The sister chromatids are exact copies produced by semi-conservative replication and are held together by cohesin proteins.
在 S 期 DNA 复制后，每条染色体由两条相同的姐妹染色单体组成，它们在着丝粒处相连。姐妹染色单体是半保留复制产生的精确拷贝，并由黏连蛋白保持在一起。
During the G1 phase of interphase, each chromosome is a single DNA molecule. After S phase and before anaphase of mitosis, it is a duplicated chromosome containing two DNA molecules. This distinction is important when counting chromosomes and DNA molecules in exam questions.
在间期的 G1 期，每条染色体是一条 DNA 分子。在 S 期之后、有丝分裂后期之前，它是一条复制后的染色体，含有两条 DNA 分子。在考试题目中统计染色体数和 DNA 分子数时，这一区别很重要。
6. Karyotypes and Chromosome Number | 核型与染色体数目
A karyotype is an organised profile of an individual’s chromosomes, prepared by staining dividing cells, photographing them under a microscope and arranging the homologues in order of size and centromere position. Human somatic cells have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
核型是个体染色体的有序图谱，通过染色分裂细胞、在显微镜下拍照并按大小和着丝粒位置排列同源染色体而制备。人类体细胞有 46 条染色体：22 对常染色体和 1 对性染色体。
The diploid number is written as 2n. It is characteristic of a species, as shown below:
二倍体数目写作 2n。它是物种的特征，如下表所示：
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