BiologyChapter 13: Chromosome and DNA
Chromosomes
A chromosome is DNA plus protein (chromatin packed).
Learning objectives
- Explain chromosomes in clear biological language.
- Distinguish 1. number from 2. structure.
- Use the key terms to answer a short Federal Board exam question.
Definition
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.
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.
Relationship
- 46 = 23 pairs
Overview
Count, then pack:
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
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).
In simple words
Karyotype names. Not a diagnostic-lab protocol.
Key points
- 46 = 23 pairs
- homologous
- 2n vs n
- chromatid vs chromosome word
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Chromosomes
Sticky Notes · Quick Revision
Core Idea
- 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.
1. 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).
2. 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.
Exam Example
- 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 Misconception
- Do not treat 1. NUMBER and 2. STRUCTURE as the same idea; each explains a different part of the topic.
FBISE Exam Check
- Define chromosomes, explain the two key parts, and use the correct biological terms.
- Exam reminder: Exam: homologous vs sister.
60-second revision
- 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.
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