Chromosomes

Chromosomes

What is it?

A chromosome is DNA plus protein (chromatin packed). Diploid humans: 46, 23 pairs. Homologous pair: same genes, maybe different alleles. Autosomes 1-22; sex pair XY or XX. Haploid gamete: 23. Centromere, chromatid, karyotype named. Chromosome number is a species fact (onion, fruit fly if the book lists them). Chromosomes carry genes in a linear order. Homologous pairs carry the same genes but possibly different alleles. Karyotypes show chromosome number and major abnormalities. Sister chromatids are identical copies; homologous chromosomes come from each parent.

In simple words

• A chromosome is DNA plus protein (chromatin packed).
• 1. NUMBER: 2n and n.
• 2. STRUCTURE: Before division, DNA copies; two sister chromatids join at a centromere.
• Exam reminder: Exam: homologous vs sister.

Important terms

CHROMOSOMES: A chromosome is DNA plus protein (chromatin packed).

1. NUMBER: 2n and n.

2. STRUCTURE: Before division, DNA copies; two sister chromatids join at a centromere.

What you should learn

• Explain chromosomes in clear biological language.
• Distinguish 1. number from 2. structure.
• Use the key terms to answer a short Federal Board exam question.

Examples

• Somatic 2n
• Gamete n
• Karyotype picture

NUMBER

2n and n. Mitosis keeps 2n. Meiosis makes n. Aneuploidy: extra or missing (Down syndrome trisomy 21 named as an example, respectfully). Human: 2n = 46 (23 pairs). Autosomes 1–22; sex chromosomes XX or XY. Gametes: n = 23. Down syndrome: trisomy 21 (three copies of chromosome 21).

STRUCTURE

Before division, DNA copies; two sister chromatids join at a centromere. Telomere named as a cap. Histones named as spools. Gene is a stretch of DNA.

Full school-level explanation

A chromosome is DNA plus protein (chromatin packed). Diploid humans: 46, 23 pairs. Homologous pair: same genes, maybe different alleles. Autosomes 1-22; sex pair XY or XX. Haploid gamete: 23. Centromere, chromatid, karyotype named. Chromosome number is a species fact (onion, fruit fly if the book lists them). Chromosomes carry genes in a linear order. Homologous pairs carry the same genes but possibly different alleles. Karyotypes show chromosome number and major abnormalities. Sister chromatids are identical copies; homologous chromosomes come from each parent.

Count, then pack:

WHAT — CHROMOSOMES

A chromosome is DNA plus protein (chromatin packed). Diploid humans: 46, 23 pairs. Homologous pair: same genes, maybe different alleles. Autosomes 1-22; sex pair XY or XX. Haploid gamete: 23. Centromere, chromatid, karyotype named. Chromosome number is a species fact (onion, fruit fly if the book lists them). Chromosomes carry genes in a linear order. Homologous pairs carry the same genes but possibly different alleles. Karyotypes show chromosome number and major abnormalities. Sister chromatids are identical copies; homologous chromosomes come from each parent.

WHY — Why this topic matters

CHROMOSOMES is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.

HOW — How it works

NUMBER: 2n and n. Mitosis keeps 2n. Meiosis makes n. Aneuploidy: extra or missing (Down syndrome trisomy 21 named as an example, respectfully). Human: 2n = 46 (23 pairs). Autosomes 1–22; sex chromosomes XX or XY. Gametes: n = 23. Down syndrome: trisomy 21 (three copies of chromosome 21). Examples: Somatic 2n; Gamete n; Karyotype picture. STRUCTURE: Before division, DNA copies; two sister chromatids join at a centromere. Telomere named as a cap. Histones named as spools. Gene is a stretch of DNA. Examples: One chromosome, two chromatids after S; Locus = address; Stain bands named if drawn. Exam: homologous vs sister. Homologous: one from each parent, same genes. Sister chromatids: copies of one chromosome. Sisters are identical (ignore rare copy error).

NUMBER

2n and n. Mitosis keeps 2n. Meiosis makes n. Aneuploidy: extra or missing (Down syndrome trisomy 21 named as an example, respectfully). Human: 2n = 46 (23 pairs). Autosomes 1–22; sex chromosomes XX or XY. Gametes: n = 23. Down syndrome: trisomy 21 (three copies of chromosome 21).

Remember

• 2n vs n

Examples

• Somatic 2n
• Gamete n
• Karyotype picture

STRUCTURE

Before division, DNA copies; two sister chromatids join at a centromere. Telomere named as a cap. Histones named as spools. Gene is a stretch of DNA.

Remember

• chromatid vs chromosome word

Examples

• One chromosome, two chromatids after S
• Locus = address
• Stain bands named if drawn

Remember

• 46 = 23 pairs
• homologous
• parent vs copy
• names only
• 2n vs n
• chromatid vs chromosome word
• 46 = 23 pairs
• homologous

How the ideas connect

Exam: homologous vs sister. Homologous: one from each parent, same genes. Sister chromatids: copies of one chromosome. Sisters are identical (ignore rare copy error).

Deeper understanding

Go beyond memorising Chromosomes. Connect the definition to reasons, relationships, exceptions, and exam wording.

How the ideas connect

Exam: homologous vs sister. Homologous: one from each parent, same genes. Sister chromatids: copies of one chromosome. Sisters are identical (ignore rare copy error).

Common mistakes

Exam focus

Core Idea

1. NUMBER

2. STRUCTURE

Exam Example

Exam focus

Quick revision

• 46 = 23 pairs
• homologous
• parent vs copy
• names only
• CHROMOSOMES: A chromosome is DNA plus protein (chromatin packed).
• 1. NUMBER: 2n and n.

Karyotype names. Not a diagnostic-lab protocol.

For long answers on Chromosomes: define clearly → explain the main parts → give one example → state one exam caution.