Chapter 19 – Structure of Chromosomes and Genes

Abstract

The three parts of nucleotide are as follows:

Chapter 19 Structure of Chromosomes and Genes

Jane Ding

1 Structure of DNA/RNA

The three parts of nucleotide are as follows:

  1. 1. Nitrogen-containing bases:

    1. ◦ purine:

      1. — fused nitrogen-containing ring

      2. — include adenine and guanine

    2. ◦ pyrimidine:

      1. — single nitrogen-containing ring

      2. — include cytosine, thymine and uracil

    3. ◦ DNA: adenine, guanine, cytosine and thymine

    4. ◦ RNA: adenine, guanine, cytosine and uracil

  2. 2. Pentose (5-carbon sugar)

    1. ◦ DNA: deoxyribose

    2. ◦ RNA: ribose

  3. 3. Phosphate: linked to pentose by phosphodiester bond

2 Nucleotide

  • basic structural unit of DNA/RNA:

    1. ◦ DNA: 100 million nucleotides; double-stranded helix

    2. ◦ RNA: 100–1000 nucleotides, single stranded

  • triplet nucleotide = codon = produces a single amino acid:

    1. ◦ one amino acid can have several codons

    2. ◦ one codon can produce only one amino acid

  • two ends:

    1. ◦ five-prime end: phosphate attaches to C5 of pentose

    2. ◦ three-prime end: hydroxyl attaches to C3 of pentose

  • base pairing:

    1. ◦ A-T (two weak hydrogen bonds, DNA)

    2. ◦ C-G (three weak hydrogen bonds, DNA)

    3. ◦ A-U (three weak hydrogen bonds, RNA)

3 Types of RNA

  • messenger RNA (mRNA): carries codon

  • transfer RNA (tRNA): carries anticodon and single amino acid

  • ribosomal RNA (rRNA): assembles amino acids

4 DNA Replication

  1. 1. DNA double helix unwinds as helicase breaks down hydrogen bonds and forms replication fork

  2. 2. DNA polymerase adds nucleotides to the leading strand (oriented 3-prime to 5-prime)

    1. ◦ polymerase travels from 5-prime to 3-prime to form a new strand continuously

  3. 3. DNA polymerase adds nucleotides to lagging strand (oriented 5-prime to 3-prime)

    1. ◦ polymerase travels from 5-prime to 3-prime to form a new strand, but in segments (called Okazaki fragments)

    2. ◦ DNA ligase joins the Okazaki fragments together

  4. 4. This process is performed at many points on DNA simultaneously to save time

5 Process of DNA Synthesising Protein

  1. 1. Transcription: DNA is copied into mRNA

  2. 2. Gene splicing

  3. 3. Translation: mRNA translates into amino acids into protein

  4. 4. Protein structure formation

  5. 5. Posttranslational modification

6 Transcription

  1. 1. DNA strands unwind, and the DNA template strand opens up

  2. 2. RNA polymerase adds nucleotides from 5-prime to 3-prime onto coding strand

    1. ◦ uracil replaces thymine in the process

  3. 3. DNA strands reanneal after mRNA is released

  4. 4. This process is performed at many points on DNA simultaneously

7 Gene Splicing

  • removes introns (noncoding sequences) and joins exons (protein-coding sequences)

  • alternative splicing: single gene translated into many proteins with different functions

8 Translation

  1. 1. tRNA carries anticodon to pair onto codon on mRNA

  2. 2. tRNA releases amino acid as anticodon attaches to codon

  3. 3. Ribosome (containing rRNA + proteins) carries the growing polypeptide and releases rRNA

  4. 4. Ribosome falls off after polypeptide completed

  5. 5. This process occurs at many points on mRNA simultaneously to save time

9 Protein Structure Formation

  1. 1. Primary structure: initial polypeptide strand

  2. 2. Secondary structure: folding into chains (alpha chain) or sheets (beta sheet) via hydrogen bonds

  3. 3. Tertiary structure: folding into final structure via hydrophobic interaction + disulphide bonds

  4. 4. Quaternary structure: multiple polypeptides combine into multimeric proteins

10 Posttranslational Modification

  • acetylation (add acetyl group): affects lifespan of protein

  • phosphorylation (add phosphate group): affects activity of protein

  • glycosylation (add carbohydrate chain)

11 Chromosome

  • cells and number of chromosomes:

    1. ◦ somatic cells (most of the body): 46 chromosomes or 23 pairs (diploid); identical in body

    2. ◦ germ cells or gametes (ovum and sperm only): 23 chromosomes (haploid); nonidentical due to recombination

    3. ◦ autosome: first 22 pairs (numbered 1–22); numbering based on decreasing chromosome length

    4. ◦ sex chromosomes: twenty-third pairs are X and Y: XX (female), XY (male)

  • shape of chromosomes

    1. ◦ short arm of chromosome: p arm

    2. ◦ long arm of chromosome: q arm

    3. ◦ metacentric chromosome: short and long arm equal length (centromere near middle of the chromosome)

    4. ◦ submetacentric chromosome: one arm longer than the other (centromere closer to one side of the chromosome)

    5. ◦ acrocentric chromosome: one arm much longer than the other (centromere at top of the chromosome)

    6. ◦ isochromosome chromosome: unbalanced, with duplication of the arms, which are mirror images resulting in two copies of either the long arm or the short arm (simultaneous duplication and deletion of genetic material)

    7. ◦ chromatid: single strand of a chromosome (chromosomes in somatic cells come in pairs)

    8. ◦ centromere: special DNA sequence where the two sister chromatids link

    9. ◦ telomere: repetitive nucleotides at the end of chromosomes; protects end of chromosome (like plastic tips on shoelaces)

    10. ◦ histone: protein that DNA wraps around to condense into chromosomes (like spools for thread)

12 Genes

  • definition: stretch of DNA sequence, which codes for a specific protein

  • human genome: 20 000 genes

Table 19.1 Cell division

Stage Description
Interphase

  • Gap 0 (G0): quiescent phase

  • Gap 1 (G1): cell growth, protein production

  • S phase: DNA replication

  • Gap 2 (G2): cell growth, protein production

Prophase Chromosomes condense
Metaphase Mitotic spindles captures all chromosomes aligned at centre of cell
Anaphase Sister chromatids separate from each other to opposite poles
Telophase

  • Nuclear envelop breaks down and reforms around daughter chromosomes

  • Cytokinesis occurs: cytoplasm division to form daughter cells

13 Mitosis

  • occurs in somatic cells

  • the aim is to produce identical diploid cells for cell reproduction

  • cytokinesis: the process of division into two cells; occurs in anaphase and telophase (see Table 19.1)

1 parent cell (23 pairs of chromosomes) [2 n] → 1 parent cell duplication → each paired chromatid separates to each daughter cell → 2 identical daughter cells (each with 23 pairs of chromosomes) [2 n + 2 n]

14 Meiosis

  • occurs in germ cells (gametes) only

  • the aim is to produce haploid cells, so that after fertilisation there will be 46 chromosomes (instead of 92 chromosomes)

  • meiosis I and meiosis II

14.1 Meiosis I

[n = haploid, 2 n = diploid]

1 parent cell (23 chromosomes) [n] → 1 parent cell duplication + recombination [2 n] → each paired chromatid moves into each daughter cell → 2 nonidentical daughter cells (each with 23 chromosomes) [n + n]

14.2 Meiosis II

2 nonidentical daughter cells (each with 23 chromosomes) [n + n] → splitting of paired chromatids into single chromatids, which moves into each final daughter cell → 4 nonidentical final daughter cells (each with 23 chromosomes) [n + n + n + n]

15 Spermatogenesis

  • produces four spermatozoa

Spermatogonium [n] → primary spermatocyte [2 n] → secondary spermatocyte (end of meiosis I) [n + n] → spermatids (end of meiosis II) [n + n + n + n] → spermatozoa (after differentiation) [n + n + n + n]

16 Oogenesis

  • produces one ovum and three polar bodies

  • polar bodies cannot fertilise (see Table 19.2)

Table 19.2 Stages in oogenesis

Oogenesis Arresting stage
Primary oocyte Prophase I (until puberty)
Secondary oocyte Metaphase II (until fertilisation)

Oogonium [n] → primary oocyte [2 n] → secondary oocyte + 1 polar body (end of meiosis I) [n + 1 polar body] → secondary oocyte + 3 polar bodies (end of meiosis II) [n + 3 polar bodies] → ovum + 3 polar bodies (after differentiation) [n + 3 polar bodies]

17 Chromosome and Gene Abnormalities

See Table 19.3.

Table 19.3 Chromosomal and gene abnormalities

Chromosomal abnormalities Gene abnormalities

  • Nondisjunction

  • Translocation

    1. ◦ Reciprocal

    2. ◦ Robertsonian

  • Microdeletion

  • Missense mutation

  • Truncating mutation: frameshift, nonsense

  • Splice site mutation

  • Whole gene deletion

  • Partial gene deletion: exon deletion

  • Triplet repeat expansion

18 Nondisjunction

See Tables 19.4, 19.5 and 19.6.

  • due to mistake in meiosis I or meiosis II

  • leads to trisomy and monosomy

  • risk increases with maternal age

Table 19.4 Nondisjunction

Meiosis I Meiosis II
Mistake Uneven spreading of homologous chromosomes One pair of homologous chromosomes did not split
End of meiosis I

  • Daughter cell 1: 3 homologous split pairs

  • Daughter cell 2: 1 homologous split pair

  • Daughter cell 1: 1 homologous unsplit pair, 1 homologous split pair

  • Daughter cell 2: 1 homologous split pair

End of meiosis II

  • Final daughter cell 1: 3 chromosomes

  • Final daughter cell 2: 3 chromosomes

  • Final daughter cell 3: 1 chromosome

  • Final daughter cell 4: 1 chromosome

  • Final daughter cell 1: 3 chromosomes

  • Final daughter cell 2: 1 chromosome

  • Final daughter cell 3: 2 chromosomes

  • Final daughter cell 4: 2 chromosomes

Table 19.5 Age and risk of nondisjunction

Age Risk
20 1/1500
30 1/900
34 1/500
36 1/300
40 1/100
42 1/60
45 1/30

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Related posts:

  1. Chapter 2 – The Breast
  2. Chapter 3 – Anatomy of the Pelvis in Obstetrics
  3. Chapter 11 – Early Fetal Embryology and Placental Development
  4. Chapter 16 – Physiology of Pregnancy and Labour

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Dec 29, 2020 | Posted by in GYNECOLOGY | Comments Off on Chapter 19 – Structure of Chromosomes and Genes

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