Biology 9 Lesson 8: Chromosomes
1. Theoretical Summary
1.1. Features of the chromosome set
Chromosomes in prokaryotes are a circular naked nucleotide acid molecule located in a non-membranous region of the nucleus.
– In somatic cells (somatic cells), chromosomes (chromosomes) exist in homologous pairs (similar in shape and size). In a homologous pair of chromosomes, one is derived from the father and the other is from the mother.
– Therefore, the genes on the chromosome also exist in corresponding pairs. The set of chromosomes that contain pairs of homologous chromosomes is called a diploid set, denoted by 2n.
– In addition, in homosexual species, there is a difference between male and female individuals in a pair of sex chromosomes that are homologously denoted XX and XY.
– Cells of organisms have a specific set of chromosomes in terms of number and shape.
– Depending on the degree of extension and twisting, the length of chromosomes is different in the phases of cell division. At metaphase, the chromosomes are maximally shortened and have a length of 0.5 to 50 μm, a diameter of 0.2 to 2 μm (1 μm = 10-3 mm), and a characteristic granular shape. , rod or V-shaped.
1.2. The structure of chromosomes
a. Microscopic structure of a chromosome
– Each typical chromosome consists of 3 special nucleotide sequences:
The centromere is the binding site for the mitotic spindle to help the chromosomes move to the poles of the cell during cell division.
+ The terminal region has the effect of protecting chromosomes and helping chromosomes not stick together.
DNA replication initiation sequences are the points at which DNA replication begins.
b. Supermicroscopic structure
The basic unit that makes up a chromosome is the nucleosome.
+ Each nucleosome consists of 8 histone protein molecules wrapped around 1 ¾ rings of DNA corresponding to 146 nucleotide pairs.
+ Nucleosomes next to each other are connected by a piece of DNA to form a nucleosome (basic strand).
+ Basic fibers (11 nm) → Chromosomes (30 nm) → Chromatite (700 nm) → Chromosomes (1400 nm).
In prokaryotic cells, chromosomes usually contain only one circular, double-stranded DNA molecule.
1.3. The function of chromosomes
– Chromosomes have different functions such as: storing, preserving and transmitting genetic information through self-replication, segregation, and combination mechanisms that take place through the processes of cell division and fertilization, etc. , chromosomes are considered to be the physical basis of heredity at the cellular level.
– Chromosomes have the ability to self-replicate, so that genetic factors that regulate traits are inherited through generations of cells and organisms.
2. Illustrated exercise
The typical structure of chromosomes is most evident in which phase of mitosis? Describe that structure.
The typical structure of chromosomes is most evident in the middle phase of mitosis.
– At this stage, the chromosome consists of two sister chromatids (chromatids) attached to each other at the centromere (first isthmus) dividing it into two wings. The centromere is the point where the chromosomes are attached to the achromatic filaments in the mitotic spindle. In metaphase, the length of the chromosomes is shortened from 0.5 to 50 μm, the diameter is from 0.2 to 2 μm, and has a characteristic granular, rod or V-shaped shape.
3.1. Essay exercises
Question 1: What is NST? Explain the structure and function of the NST.
Verse 2: What are the characteristics of the species specificity of the chromosome set?
Question 3: Chromosomes are most visible during which phase of cell division? Describe the structure of the chromosome at that period.
Question 4: What is a homologous pair of chromosomes? In which cells are homologous pairs of chromosomes found?
3.2. Multiple choice exercises
Question 1: One ability of chromosomes to play a very important role in heredity is:
D. Contraction and stretching during mitosis
Verse 2: In cells in organisms, chromosomes have the form:
A. Stick figure
B. Particle shape
D. Many shapes
Question 3: During mitosis, the morphology of chromosomes can be most clearly observed at:
A. In the intermediate period
B. First period
D. Next period
Question 4: When not duplicated, each chromosome consists of:
A. a chromate
B. a single chromosome
C. a double chromosome
D. chromatid pair
Question 5: The chemical composition of NST includes:
A. Protein molecules
B. DNA molecule
C. Proteins and DNA molecules
D. Acids and bases
After completing this lesson, you should:
- State the characteristics of chromosomes.
- Describe the microscopic and ultramicroscopic structure of a chromosome.
- State the role of chromosomes in heredity.