7.1 Chromosome Structure

7.1 Chromosome Structure

A chromosome is DNA plus proteins. The DNA stores genes as a sequence of nucleotides, and the proteins fold, protect, and move that DNA. Between divisions the same material is spread through the nucleus as chromatin. Chromatin condenses when a cell divides, which shortens each chromosome and keeps long DNA molecules from tangling while the spindle pulls them. Condensation changes the packaging. It does not change which alleles the chromosome carries.

From one DNA molecule to two sister chromatids

Before S phase, a chromosome is one DNA molecule. That molecule is a double helix with one centromere, the constricted region used to attach the chromosome to the spindle. Chromosome number is a count of centromeres. It is not a count of DNA strands, chromosome arms, or chromatids.

During S phase the cell replicates that DNA. After replication the chromosome consists of two sister chromatids joined at the centromere. Those chromatids are copies of the original molecule. They still count as one chromosome because they share one centromere. Cohesin proteins hold the sister chromatids together along their length until the cell is ready to separate them. Sister chromatids are identical copies until crossing over in meiosis. When the chromatids finally separate, each one becomes a chromosome with its own centromere.

The centromere is a region of the chromosome, not the organelle that builds the spindle. The kinetochore is the protein complex assembled at the centromere so microtubules can bind. The ends of a linear eukaryotic chromosome are telomeres, repeated sequences that keep the ends from being treated as broken DNA. Telomeres, the centromere, and the histone proteins are parts of one chromosome. None of them is an extra chromosome.

Homologous chromosomes are a parental pair

Homologous chromosomes are not sister chromatids. A homolog pair is one maternal chromosome and one paternal chromosome that carry the same genes in the same order. They do not necessarily carry the same alleles. An allele is a version of a gene. One homolog might carry an allele for a working enzyme and the other an allele that changes that enzyme, yet both chromosomes still belong to the pair.

Homologs are separate chromosomes, each with its own centromere. DNA replication does not create them. The two parents transmit them. Sister chromatids, by contrast, are the two copies made when one chromosome is replicated. Calling a homolog a sister chromatid, or a sister chromatid a homolog, swaps two relationships the exam keeps distinct.

Diploid cells keep pairs and haploid cells keep one set

A diploid cell, written 2n, has both members of each homologous pair. A haploid cell, written n, has one set: a single chromosome of each kind. Humans are a familiar example. A typical human body cell is diploid, with 46 chromosomes organized as 23 pairs. A human gamete is haploid, with 23 chromosomes. The exam often uses a generic organism instead. If that organism is diploid with 8 chromosomes, the haploid number is 4. Halve the diploid number to get n, and double the haploid number to get 2n.

Histones, nucleosomes, and the karyotype

Eukaryotic DNA is much longer than the nucleus that holds it. Histones package it. Two copies each of histones H2A, H2B, H3, and H4 form an eight-protein core. DNA wraps that core to form a nucleosome, the repeating packaging unit of chromatin. Linker DNA runs between nucleosomes, and histone H1 commonly binds that linker and helps the chain fold. Further folding packs nucleosomes into a shorter chromosome that a light microscope can resolve at division.

A karyotype is an ordered laboratory display of condensed chromosomes, usually taken from a metaphase image and arranged by size, centromere position, and banding pattern. Biologists use it to inspect chromosome number and large structural changes. Preparing a karyotype does not move chromosomes inside a living cell, and it is not a phase of the cell cycle.

The cell cycle copies DNA only in S phase

The cell cycle is the ordered sequence of growth, DNA replication, and division.

G1, S, G2, and M

• G1, the first gap, is growth and ordinary cell work. Chromosomes in a typical G1 nucleus are unreplicated.
• S phase is synthesis. The cell replicates each nuclear DNA molecule, and each chromosome becomes two sister chromatids. This is the only part of the cycle in which those chromosomes are copied.
• G2, the second gap, is preparation. The cell checks that replication finished and builds proteins the spindle will need. Chromosomes stay duplicated throughout G2.
• M phase is mitosis. Mitosis sorts chromosomes that were duplicated back in S phase. It does not replicate them. Cytokinesis usually follows and divides the cytoplasm.

G1, S, and G2 together are interphase. Many cells spend most of their lives there. Some leave the cycle in a non-dividing state often called G0. A typical resting cell holds unduplicated chromosomes and is not in mitosis.

A counting check

Consider a diploid cell with 8 chromosomes in G1. It has 8 DNA molecules and 8 centromeres. After S phase it still has 8 chromosomes, because it still has 8 centromeres, but it has 16 chromatids and 16 DNA molecules. A question that reports 16 separate chromosomes at the end of S phase has counted chromatids as if they had already separated. They have not.

A human body cell follows the same count: 46 chromosomes in G1, and still 46 chromosomes with 92 chromatids after S phase. The chromosome count rises only when sister chromatids separate.