CHROMOSOMES Flashcards
Define the term chromosome
Chromosomes are highly coiled, ribbon like structures formed by the condensation of chromatin fibres during cell divison
Who discovered chromosomes?
Walther Flemming
Who first studies the structure of chromosomes?
Rosalind Franklin
Who proposed the double helical structure of DNA?
Watson and Crick
What is the composition of chromatin fibre?
DNA - 40%
Histones - 60%
What is the structure of DNA?
Double helical macromolecule
What are nucleotides?
Nucleotide is the structural and functional unit of DNA.
DNA is composed of repeating nucleotides.
What are nucleotides made up of?
pentose sugars, nitrogenous bases, phosphate and hydrogen bonds
What are the 4 types of nitrogenous bases in DNA?
Adenine, guanine, thymine and cytosine
What are the different types of nitrogenous bases in RNA?
Adenine, guanine, cytosine and uracil
Define the term histone
Histones are proteins which help in the coiling and packaging of DNA into structural units called nucleosome
What are nucleosomes?
1 strand of DNA winds around a core of 8 histone proteins and is called histone octamer or nucleosomes
What are the structural and functional units of chromosomes?
Nucleosomes
What are the 2 types of nucleic acids?
DNA & RNA
Define centromere
Centromere is the point of attachment of 2 sister chromatids that appears as a small constricted region
What are the functions of centromere?
- Centromere helps in attaching the sister chromatids to the spindle fibres
- they help in the detaching of the sister chromatids during the anaphase stage
What is the structure of a chromosome?
A chromosome is composed of 2 sister chromatids that are joined at the centromere
Define the term chromatid
Chromatid is one vertical half of a duplicated chromosome
What are sister chromatids?
Sister chromatids are two identical chromatids joined at the centromere
What are genes?
Genes are specific sequences of nucleotides on a chromosome that encode particular proteins and are expressed in the form of some particular feature on the body
What is the need for new cells?
- growth
- repair
- replacement
- reproduction
Define cell cycle
Cell cycle is a series of events leading to the duplication of DNA and division of the cell to produce two daughter cells
What are the 2 phases of the cell cycle?
- interphase
- mitosis
What is the interphase?
The interphase is a non dividing phase of the cell where the relatively small daughter cells prepare for the next cell division and grow to the same size as the mother cell
What are the 3 phases of the interphase?
- Growth phase 1
- Synthesis phase
- Growth phase 2
What are the 2 paths that a cell may follow in Growth phase 1?
The cell may withdraw from the cell cycle during Growth phase 1 and enter into a Resting phase or continue to the Synthesis phase
What are the changes in the cell during Growth phase 1?
- RNA and proteins are synthesized
- Chloroplasts or mitochondria duplicate
- Cytoplasm increases
What is synthesized in the Synthesis phase?
DNA is synthesized
chromosomes are duplicated
What is synthesized during Growth phase 2?
RNA and proteins
How does DNA duplicate?
The DNA opens at one end, making the 2 strands free, to which new strands form
What are the 2 types of cell division?
- Mitosis
- Meiosis
What is mitosis?
Mitosis is the cell division leading to the production of diploid cells
- one parent cell divides into 2 identical daughter cells
What are the 2 phases of mitosis?
- Karyokinesis
- Cytokinesis
What is karyokinesis?
Karyokinesis is the division of the nucleus of the cell
All the nuclear changes that take place in the cell during cell division can be called karyokinesis
What are the 4 phases of karyokinesis?
- prophase
- metaphase
- anaphase
- telophase
What is cytokinesis?
The division of cytoplasm is called cytokinesis
What changes take place during prophase?
- The nuclear membrane disappears
- nucleolus dissapears
- centrioles move towards opposite poles (animal cell)
- spindle fibres are formed
- chromosomes become distinct
What is an Aster?
Centrioles are surrounded by radiating rays and are termed as aster
What changes take place in the cell during metaphase?
- chromosomes line up in one plane on the equator
- chromosomes attach to the spindle fibres with the help of their centromeres
What changes take place in the cell during anaphase?
- centromere splits
- sister chromatids separate
- sister chromatids move to opposite poles
- a furrow starts developing ( animal cell )
What changes take place in the cell during telophase?
- 2 nucleus form
- spindle fibres disappear
- nuclear membrane forms
- chromatids thin out to become chromatin fibres
- furrow deepens ( animal cell )
cell plate is laid down ( plant cell )
Asters are formed in which cell?
(a) animal cell
(b) plant cell
(a) animal cell
How are spindle fibres formed in animal cells?
with the help of centrioles and asters
How are spindle fibres formed in plant cells?
with the help of microtubules
How does cytokinesis occur in animal cell?
By furrowing of cytoplasm
How does cytokinesis occur in plant cell?
By formation of cell plate
What is the significance of mitosis?
- growth
- repair
- replacement
- asexual reproduction
- same number of chromosomes is maintained in the daughter cells
Why do mitochondria and chloroplasts divide on their own?
Mitochondria and chloroplasts have their own DNA and ribosomes which help them produce particular proteins. They divide by simple fission.
What is meiosis?
Meiosis is the cell division leading to the production of haploid cells or gametes - sperms and eggs
What is the significance of meiosis?
- number of chromosomes are halved - haploid
- mixing up of genes occurs
How does mixing up of genes take place during meiosis?
- during the first reduction division, due to the separation of the chromosomes from the homologous pairs
- by crossing over
- random assortment of chromosomes between the gametes
Why is the chromosome number halved during meiosis?
As the diploid (2n) number of chromosomes will be restored on fertilization
What is crossing over?
The exchange of genetic material during meiosis between a pair of non sister chromatids from a homologous pair of chromosomes
What is chiasma?
The point of attachment between 2 non sister chromatids of a pair of homologous chromosomes during meiosis
Where does mitosis occur?
Vegetative or somatic parts of the plant
Where does meiosis occur?
In the reproductive cells
Why does meiosis occur?
To form gametes
How many daughter cells are formed in mitosis?
2 daughter cells
How many daughter cells are formed in meiosis?
4 daughter cells
How many divisions occur in
a) mitosis
b) meiosis
a) 1 division
b) 2 divisions
How many chromosomes are passed on in
a) mitosis
b) meiosis
a) 2n - diploid
b) n - haploid
When does it occur?
a) mitosis
b) meiosis
a) throughout life
b) only during reproductively active age
How are the daughter cells with respect to their parent cells after mitosis?
daughter cells are identical to parent cells
What are homologous chromosomes?
Homologous chromosomes are chromosomes that are of the same length and centromere positioning.
One chromosome is inherited from each parent
How many hydrogen bonds is adenine paired with thymine with?
2 hydrogen bonds
How many hydrogen bonds is guanine paired with cytosine with?
3 hydrogen bonds
