DNA structure, cell and nuclear division Flashcards
starting from page 19
what are the functions of centromeres?
- during cell division, centromeres are adhesion points for sister chromatids in a chromosome
- they are aslo the site of assembly of kinetochore proteins, which is where the spindle fibres attach to during cell division
- the attachement of spindle fibres to kinetochore enables:
- accurate alignment of chromosomes along the metaphase plate during metaphase of mitosis and meiosis
- accurate segragatio of homologous chromosomes during anaphase of meiosis I
- accurate SEGREGATION of sister chromatids to form daughter chromosomes during anaphase of mitosis and anaphase of meiosis II when centromere divides
- during anaphase of mitosis and meiosis
- chromosomes are pulled to opposite poles of the cell with the centromere leading
- enabling a complete set of chromosomes to be allocated to each daughter cell
homologous chromosomes
what is polyploidy?
- in most organisms, chromosomes in cells can be divided into 2 sets
- however the nuclei of cells of some organisms may contain more than 2 sets of chromosomes
- this condition is known as polyploidy which is common in plants
- in cells hwich have 2 sets of chromosomes, the chromosomes occur in homologous pairs and are known as homologous chromosomes
- each member of the homologous chromosome is called homologue
homologous chromosomes
what are the characteristics of homologous chromosomes (except sex chromosomes)
- homologous chromosomes are a pair of chromosomes which are similar in length, size, shape and centromere position
- homologous chromosomes carry genes which determine and control the same characteristics
- however, they are not necessarily genetically identical in the gene that they carry
- they may have different alleles
- they have identical sequences of gene loci
- ie. the same number of genes arranged in the same linear order
- each pair of homologous chromosome is made up of one maternal and one paternal chromosome
- they can form bivalents during meiosis and appear as double structures consisting of two genetically identical sister chromatids
what are haploid cells?
- any cell with 1 complete set of chromosomes is called a haploid cell
- haploid cells contain one member of each homologous pair of chromosomes
- it has hald the normal number of chromosomes as a diploid cell
- these cells have a haploid number of chromosomes
- the gametes of human beings are haploid cells
what are diploid cells?
- any cell with 2 complete sets of chromosomes is called a diploid cell
- diploid cells contain chromosomes that are in pairs, with one from each parent
- these cells have a diploid number of chromosomes 2n
- the somatic cells are any cells other than gametes and those involved in gamete formation
haploid and diploid cells in humans
- in each human somatic cell, there are 46 chromosomes
- the 46 chromosomes are 2 sets of 23 chromosomes- a maternal and paternal set
- the 2 sets of 23 chromosomes consist of 22 pairs of autosomes and a single pair of sex chroomosomes
- therefore, human somatic cells are diploid and the diploid number of human somatic cells is 46
what are the functions of DNA?
- DNA molecules contain genes- heredity units that contain genetic information that determine an organism’s characteristics
- genes on DNA provide information for the synthesis of RNA via transcription and through RNA, controls protein synthesis (translation)
- proteins are the building blocks of cells and perform many important functions like enzymes, receptors, transport proteins
- the functions performed by these proteins determine an organism’s characteristics
-DNA undergoes replication prior to cell division so that genetic information can be transmitted to the next generation
describe DNA replication before nuclear division
- a dividing cell replicates its DNA to produce two complete sets of genome which are then allocated to two daughter cells during cell and nuclear division
- DNA replication is necessary for accurate passing on of genetic information present in the parent cell to daughter cells after cell division
- this ensures all daughter cells are genetically identical
- ie, daughter cells receives a complete set of genome with the same number and type of chromosomes
- DNA replication must occur before nuclear division
- nuclear division occurs before cytokinesis to ensure that DNA is passed down from one generation of cells to the next with high fidelity
what is the cell cycle?
- the cell cycle can be defined as the period from the formation of a cell by division to the point that the cell itself divides into two genetically identical daughter cells
- most type of cells never divide again after they have grown and become specialised
the cell cycle consists of two phases: interphase and the Mitotic (M) PHASE
**interphase comprises 3 sub-phase
**- G1 phase
- S phase (DNA replication occurs)
- G2 phase
**the mitotic (M) phase **
- mitosis (NOT MEIOSIS
- cytokinesis
what is cell cycle control?
- the order of sequential events that occur in the cell cycle must be strictly adhered to
- serious genetic damage can occur if cells progress to the next phase of the cell cycle before the previous phase is properly completed
- to do so, the cell has a control system comprising a group of signalling and regulatory proteins foudn in the cytoplasm
what are the cell cycle control checkpoints for?
the cell cycle is controlled at three checkpoints
- at the G1 checkpoint
- at the G2 checkpoint
- at the metaphase (M) checkpoint
where are checkpoints needed?
- preven premature progression of the cell cycle
- provides time for cell machinery to be repaired should there be any damage
what is interphase?
- it is an important period in the cell cycle whent he cell is not dividing
- during interphase, the cell prepares for cell division
- interphase is the longest phase which takes up about 90% of the cell cycle
- it consists of G1, S and G2 phases
what happens during G1 phase and G1 Checkpoint?
G1 PHASE:
1. intensive cellular synthesis occurs
- synthesis of organelles
- synthesis of the following molecules:
> ATP, to build up a large energy store
> RNA
> proteins
- DNA is in the uncondensed form as chromatin fibres
G1 CHECKPOINT:
1. checks that:
- cell size is adequate
- there is sufficient nutrients that are availible to support daughter cells
- growth factors (extracellular signal proteins that stimulate a cell to grow or divide) are present
- it decides whether the cell should continue in the G1 phase or initiate the molecular processes that signal the beginning of S phase
- if the cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2 and M phases and divide
- if not, it will exit the cycle and switch into a non dividing state called the G0 phase
