Fertilized egg by a sperm cell
Cell division to create more cells: growth
development of cells
partially coiled DNA with histones (proteins that DNA wrap around). Normal state of DNA when cell is not dividing (meiosis or mitosis)
large bodies which are composed of one long strand of DNA, supercoiling?
or deoxyribonucleic acid is a very special molecule which carries information of the species, and individual traits
are lengths of DNA that carry a code for the structure of proteins. These proteins give us our characteristics. Each chromosome in a pair carries the same sequence of genes therefore we have pairs of genes. For each protein or characteristic we possess, we have a pair of genes. These two genes may be identical or they may be slightly different from each other – if they are not the same they are called alleles. An allele is an alternative form (version) of the gene
when two genes may be identical or they may be slightly different from each other – if they are not the same they are called alleles. An allele is an alternative form (version) of the gene.
One parent is needed. Cell division by mitosis produces two cells. These cells are identical (genetically) to the parent. this is faster than sexual reproduction, the environment supports the growth, if environment changes however, the population will get wiped out.
Parent cells have 2 sets of chromosomes (diploid cells). Humans 2n=46. Cell division by meiosis. Sex cells (gametes) produced have half the number of chromosomes (haploid cells). Humans n=23 (one from each pair found in the parent cell). Fertilization is fusion of the sex cells. The correct number of 46 chromosomes is restored in the offspring . The offspring has one set of 23 chromosomes from each parent making the full 23 pairs usually found. Advantage – meiosis is a source of variation for natural selection
- Parent cell with 4 chromosomes, for example (human cells have 46 chromosomes)
- Chromosomes are duplicated (each one with two chromatids)
(now, 4 duplicated chromosomes)
- Chromosomes in the middle of cell
- Chromatids now split to opposite cell poles
- 2 daughter cells identical to original
How are characteristics passed on?
Genes are passed from one generation to the next by reproduction
Sexual vs. Asexual Reproduction
- asexual: Mitosis, cells are genetically identical so their descendants will also be identical. This is good for environments where cell is adapted, if environment changes however, every cell will die.
- Sexual: meiosis, cells are different genetically which allows natural selection. Gametes produced must fuse together to get 46 chromosomes so there is variation.
chromosomes replicate inside cell, proteins are made and energy stores are built up (this looks like a phase of “resting”)
chromosomes coil to become visible (two chromatids), nucleus is less visible and centrioles move to opposite poles and form spindle
- when similar chromosomes pair up, this is called synapsis where each pair is a bivalent or tetrad
- late phase: nuclear envelope breaks , chromatids may cross (those places are chiasma)
- Homologous chromosomes line up and exchange DNA (crossing over creates more genetic diversity)
- bivalents(chromosome pairs) move to lie on equator of cell (middle)
- spindle fibres attach to chromosomes to move them
- homologous chromosomes are pulled apart by the breakdown of microtubules called spindle fibres
- sister chromatids remain attached
- centromeres DON’T divide unlike mitosis
- chromosomes come to lie at poles
- nuclei may reform but not usually
- may be resting phase (interkinesis) after this phase
- cytokinesis (cytoplasm divides)
- like mitosis but in two cells
- cell preps to divide again
- new spindles are made
- poles are at right angles to original poles???
- chromosomes move to lie on equator
- spindle fibers attach to sister chromatids through the kinetochore???
- sister chromatids are pulled apart by spindle fibers
- centromeres split
- chromatids collect at poles
- new nuclei form
- cytoplasm divides (cytokinesis)
- daughter cells have half the number of chromosomes (haploid)
- this results in daughter cells that are haploid and are genetically unique.
- reduction division (number of chromosomes has lessened)
- homologous chromosomes separate (some from dad, some from mom, in each cell = 23)
- vs. mitosis: there are two separations, mitosis creates genetically identical cells,
- exactly like mitosis
- separate the chromatids
- start with diploid (2n) and then haploid (n) and then chromatids separate BUT it is still 23 chromosomes with haploid number (count the # of centromeres)