BIO 101 characteristic and classification of living organisms

DAVID UMAHI FEDERAL UNIVERSITY OF HEALTH SCIENCES

BIOLOGY/BIOTECHNOLOGY DEPARTMENT
BIO 101: GENERAL BIOLOGY LECTURE
Topic: Chromosomes, Genes; their Relationship and Importance (Cont’d)
BY DR. OKO, A.O and OGBU, C.A

CHROMOSOMES STRUCTURE

  • Telomere:

    • Terminal region located at each end of the chromosome.

  • Chromomere:

    • Beadlike structure found on threads or chromonema.

    • Carries genes during cell division to the next generation.

  • Chromonema:

    • Threadlike coiled filamentous structure along which chromosomes are arranged.

    • Controls size of the chromosome & acts as a gene-bearing site.

  • Centromere/Kinetochore:

    • Primary constriction at the chromosome's center.

    • Point of attachment for chromatids or spindle fibers.

  • Chromatid:

    • Each half of a chromosome.

    • Separates during anaphase to form a separate chromosome.

  • Chromatin:

    • Complex of DNA and proteins that form chromosomes within the nucleus.

  • Pellicle:

    • Membrane surrounding each chromosome.

  • Matrix:

    • Jellylike substance inside the pellicle.

    • Both pellicle and matrix are non-genetic materials.

![Figure 2 A simplified structure of chromosomes]

TYPES OF CHROMOSOMES

  • Homologous Chromosomes:

    • Two chromosomes that are similar, at least regarding gene sequences and loci.

  • Heterologous Chromosomes:

    • Any two chromosomes that differ, particularly in terms of gene sequence and loci.

CHEMICAL COMPOSITION OF CHROMOSOME (CCC)

  • Nuclei-Protein Nature:

    • Composed of RNA, DNA, and Protein.

  • Composition Ratios:

    • 30-40% DNA, 50-65% protein, 0.5-10% RNA.

    • DNA is constant in somatic cells and halved in gametic cells.

Types of Chromosomal DNA

  1. Unique DNA (non-repetitive DNA):

    • DNA sequences found in single copies per genome.

    • Codes for proteins required in large amounts (e.g., storage proteins).

  2. Repetitive DNA:

    • Consists of DNA sequences present in hundreds to millions of copies in the genome.

    • Does not code for proteins.

    • Human genome contains approximately 30% repetitive DNA.

    • Divided into:

      • Highly Repetitive DNA

      • Moderate Repetitive DNA

  • RNA Composition:

    • Purified chromatin contains 10-15% RNA.

    • RNA types include mRNA, tRNA, and rRNA.

CCC CONTINUED

PROTEIN COMPOSITION OF CHROMOSOMES

  • Histone or Basic Proteins:

    • Constitute about 80% of total chromosomal protein.

    • Present in almost a 1:1 ratio with DNA.

    • Five fractions: 1H1, 2H2a, 2H2b, 2H3, and 2H4.

  • Non-Histone Proteins:

    • Make up to 20% of total protein mass.

    • Content varies by species.

    • Include important enzymes such as DNA and RNA polymerase.

    • Non-histone proteins are acidic, while histone proteins are basic.

CHROMOSOME NUMBERS

  • Species Variation:

    • Generally, individuals of a species share the same chromosome number.

    • Closely related species typically exhibit similar chromosome numbers.

  • Basic Chromosome Number Types:

    • Somatic Chromosome Number (2n):

    • Chromosomes that are not sex chromosomes; appear in pairs in body cells but as single in spermatozoa.

    • Gemetic Chromosome Number (n):

    • Chromosomes in haploid cells (e.g., spermatozoon and ovum) as opposed to diploid cells.

CHROMOSOME NUMBERS CONTINUED

  • Euploid:

    • Presence of whole sets of chromosomes.

    • Types include haploids (n), diploids (2n), triploids (3n), tetraploids (4n), etc.

  • Gametes:

    • Normally contain a single set of chromosomes (haploid).

  • Somatic Cells:

    • Typically contain two sets of chromosomes (diploid: 2n).

  • Polyploidy:

    • Condition of having chromosome sets in multiples of "n".

  • Aneuploidy:

    • Change in chromosome number that involves only a few chromosomes (e.g., Monosomy (2n - 1), Turner syndrome; Trisomy (2n + 1), Down syndrome, Klinefelter syndrome; Nullisomy (2n - 2); Tetrasomy (2n + 2), cat-eye syndrome, XXYY syndrome, Tetrasomy X).

  • Embryo Classification:

    • An embryo with a normal chromosome number is termed 'euploid', while one with an abnormal number is termed 'aneuploid'.

Table 1: Somatic Chromosome Number of Some Common Plants and Animals

Scientific Name

Common Name

Somatic

Gemetic

Homo sapiens

Human

46

23

Felis domesticum

Cat

38

19

Rattus norvegicus

Rat

42

21

Apis mellifera

Honey bee

32

16

Musca domestica

House fly

12

6

Drosophila melanogaster

Fruit fly

8

4

Oryza sativa

Rice

24

12

Neurospora crassa

Bread mold

14

7

CHROMOSOME SIZES

  • Variation in Sizes:

    • Size varies significantly depending on the cell division stage.

    • Interphase:

    • Chromosomes are longest and thinnest.

    • Prophase:

    • Progressive decrease in length with increased thickness.

    • Metaphase:

    • Chromosomes are thick, short, and best observed.

    • Measurements typically taken during mitotic metaphase.

CHROMOSOMES MEASUREMENTS

  • Size Range:

    • Chromosome lengths during mitotic phases range from 0.5 to 32 microns, and diameters from 0.2 to 3 microns.

    • Longest metaphase chromosomes found in Trillium (32 microns).

    • In general, plants have longer chromosomes than animals.

    • Species with lower chromosome numbers tend to have longer chromosomes compared to those with higher numbers.

    • Among plants, dicots generally have more chromosomes than monocots, although monocots have longer chromosomes than dicots.

CLASSIFICATION OF CHROMosomes CONTINUED

  • Denver’s Classification:

    • Based on total length of the chromosome into seven groups (A-G):

    • 1, 2, 3

    • 4, 5

    • 6-12

    • 13, 14, 15

    • 16, 17, 18

    • 19, 20

    • 21, 22

  • Centromere Number Classification:

    • Monocentric:

    • All 46 human chromosomes are monocentric.

    • Dicentric:

    • Resulting from Robertsonian translocation.

    • Polycentric

    • Acentric

FUNCTIONS OF CHROMOSOMES

  • Provide genetic information critical for cellular functions in organisms.

  • Protect genetic material (DNA) from damage during cell division.

  • Ensuring accurate DNA distribution to daughter nuclei during cell division.

  • Involvement in sex determination.

  • Formation of proteins and storage roles.

  • Assistance in growth, repair, reproduction, and regeneration processes.

GENE

  • The contemporary concept of the gene originated from Gregor Mendel's studies on inheritance in the 1860s.

  • Gene Definition:

    • Segments of deoxyribonucleic acid (DNA) that code for specific proteins functioning in various cell types or RNA molecules.

  • DNA undergoes transcription to mRNA, which is translated into proteins (the central dogma of molecular biology).

  • DNA serves as the blueprint for synthesizing proteins or RNA.

  • Gene size varies depending on the proteins or RNA they code for.

  • Genes function as the molecular unit of heredity; traits are gene-determined characteristics, often influenced by multiple genes.

  • Chromosomes are arranged in a specific sequence, with each gene occupying a specific location (locus) on a chromosome.

  • An allele is a variant form of a gene that occupies the same locus on each homologous chromosome.

TYPES OF GENES

  • Categorization based on gene behavior includes:

    • Basic Genes:

    • Fundamental genes causing the expression of specific traits.

    • Lethal Genes:

    • Responsible for the death of organism's possessors.

    • Multiple Genes:

    • Independent gene pairs acting together to yield a single phenotypic trait.

    • Cumulative Genes:

    • Have additive effects on other genes' actions.

    • Pleiotropic Genes:

    • Produce alterations in multiple characteristics.

    • Modifying Genes:

    • Interaction with other genes for modified effects, not producing traits independently.

    • Inhibitory Genes:

    • Suppress or inhibit the expression of other genes.

GENE ACTIONS/IMPORTANCE

  • Gene Action:

    • Refers to the influence of genes resulting in genetic character expression.

  • Genes are typically linked to enzyme production synthesized from chemical substances present in body cells through autocatalysis.

  • Gene actions contribute to the development of:

    • Pigments

    • Coloration

    • Hormones

    • Size and form

    • Protein production

    • Antigens and antibodies

  • Genes can influence significant human diseases, such as albinism and tyrosinemia.

GENE & CHROMOSOMES; APPLICATIONS/RELATIONSHIP

  • Determine physical and physiological characteristics.

  • Reside within chromosomes.

  • Occupy specific positions known as loci.

  • Arranged in a linear order in a single sequence.

  • Some contain more than two alleles, termed multiple alleles.

  • Duplication occurs accurately (replication).

  • Determine amino acid sequencing in polypeptide chains.

  • May cause genetic mutations.

Table 2: Differences between Chromosome and Gene

Chromosome

Gene

Most condensed structure of a DNA molecule with proteins

A locus on a chromosome

Always composed of DNA

Composed of either DNA or RNA

Larger mutations (homologous recombination)

Smaller mutations (DNA replication or DNA damage)

Result in chromosomal abnormalities (duplication, deletion, rearrangement, inversion)

Point mutations or frame-shift mutations (insertions/deletions)

EVALUATION

  • Define chromosomes in four different ways.

  • Explain the nature of chromosomes in prokaryotes, eukaryotes, plants, and animals.

  • What are autosomes, and how many autosomes are there in humans?

  • Describe the relationship between chromosomes and genes.

  • With diagram aid, explain homologous and non-homologous chromosomes.

  • Explain the nature of chromosomes in interphase, prophase, and metaphase of cell division.

  • List and explain the three ways chromosomes can be classified.

  • State four functions of chromosomes.

  • What are Genes? State the types of genes and explain them.

  • State the importance of Genes.

  • Provide three differences between chromosomes and genes.