DNA and Meiosis Flashcards
What is DNA made of?
NUCLEOTIDES
What do nucleotides contain?
a SUGAR (Deoxyribose), a PHOSPHATE and a BASE
DNA
A polynucleotide- made up of lots of nucleotides joined together.
What is the same/ different in every nucleotide?
Same sugar and phosphate, the base can vary.
What are the four bases?
A- adenine,T- thymine,C- cytosine,G- guanine
Structure of DNA
- DNA nucleotides join together to form polynucleotide strands.
- Nucleotides join up between the phosphate group of one nucleotide and the sugar of another, creating a SUGAR-PHOSPHATE BACKBONE.
- Two DNA polynucleotide strands join together by hydrogen bonds between the bases.
- Each base can only join with one particular partner- this is called SPECIFIC BASE PAIRING.
- Adenine pairs with Thymine, and Guanine with Cytosine.
- Two strands wind up to form the DNA double helix. Deoxyribose molecules alternates with phosphate to form the strands and the bases form rungs.
How is DNA adapted to carry out its functions? (6)
- Base pairs within helical cylinder so genetic info is somewhat protected from being corrupted by outside chemical and physical forces.
- DNA molecules v. long and are coiled very tightly, so lots of genetic info can fit into a small space in cell nucleus.
- Large molecule- hold lots of info
- Have a paired structure so makes it easier to replicate itself. SELF REPLICATION.
- Double helix makes it a v. stable cell and can pass from generation to generation without change.
- Two separate strands are joined only with H bonds, allows them to separate during DNA replication and protein synthesis.
What do the organic bases contain?
NITROGEN
What are the two types of base?
- Double ring structure (Adenine and Guanine) have longer molecules.
- Single ring structure (Thymine and Cytosine) have shorter molecules. Rungs of DNA made of one of each type.
How are the base pairs bonded?
Adenine and thymine- 2 hydrogen bonds
Cytosine and Guanine- 3 hydrogen bonds
They are COMPLEMENTARY to each other.
Eukaryotic DNA
LINEAR and associated with proteins.
- DNA molecules exist as chromosomes
- Really long DNA molecule, wound up to fit in nucleus
- DNA molecule wound around proteins which help support it
- The DNA is then coiled up very tightly to make a compact chromosome.
CHROMOSOME
A thread like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next.
Prokaryotic DNA
- DNA shorter and circular
- Carried as chromosomes
- Isn’t wound by proteins
- it condenses to fit in the cell by supercoiling.
Chromosome structure
ONLY VISIBLE AS DISTINCT STRUCTURES WHEN A CELL IS DIVIDING.
- When visible they appear as two threads, joined at a single point. (centromere)
- DNA in chromosomes is held on position by proteins, highly coiled and folded.
- DNA- protein complex then coiled.
- Coil in turn is looped and further coiled before being packed into the chromosome.
A LOT OF DNA IS CONDENSED INTO A SINGLE CHROMOSOME.
HOMOLOGOUS CHROMOSOMES
a pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are NOT necessarily identical, however, as individual ALLELES of the same GENE may vary e.g. one chromosome may carry allele for blue eyes, the other the allele for brown eyes. Capable of pairing during meiosis.
Why do chromosomes occur in pairs?
Sexually produced organisms, such as humans, are the result of the fusion of sperm and egg, each of which contributes a set of chromosomes to the offspring.
HAPLOID
Term referring to cells that contain only a single copy of each chromosomes.n e.g. GAMETES
DIPLOID
A term applied to cells in which the nucleus contains two sets of chromosomes.
GENE
section of DNA on a chromosome coding for one or more polypeptide.
ALLELE
One of a number of alternative forms of gene. E.g. blue eyed green eyed etc.
What do genes do?
INSTRUCTIONS FOR MAKING PROTEINS
How do genes code for proteins?
Proteins made from amino acids.
Different proteins have different number and order of amino acids.
It’s the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein.
Each amino acid is coded for by a sequence of THREE BASES (triplet) in a gene.
Different sequences of bases code for different amino acids.
E.g. GTCTCATCA | | ——> read this way GTC= valine TCA= serine Valine- Serine- Serine
Introns
In eukaryotes much of the nuclear DNA does not code for amino acids. Can occur within genes and as multiple repeats between genes.
GAMETE
reproductive cell that fuses with another gamete during fertilisation. HAPLOID
Zygote
Fertilised egg divides and develops into new organism
MEIOSIS
the type of nuclear division in which the number of chromosomes is halved. DIPLOID—>HAPLOID
Process of meiosis.
MEIOSIS 1: the chromosomes arrange themselves into homologous pairs.
Their chromatids wrap around each other.
Equivalent portions of these chromatids may be exchanged in a process called CROSSING OVER.
These homologous pairs are then separated, halving the chromosome.
MEIOSIS 2: The chromatids move apart. At the end of meiosis 2, 4 cells have been formed.
CHROMATID
One of two copies of a, chromosome that are joined together by a single centromere prior to cell division
LOCUS
the position of a gene on a chromosome or DNA molecule.
INDEPENDENT SEGREGATION of homologous chromosomes.
The combination of chromosomes that goes into the daughter cell at meiosis one is random because homologous pairs are lined up randomly.
How meiosis produces GENETIC VARIATION.
Each member of a homologous pair of chromosomes has exactly the same genes so determine same characteristics. However alleles of these genes may vary.
The random distribution, and consequent independent assortment, of these chromosome therefore produces new genetic combinations.
Describe the process of crossing over
- Chromatids of each pair become twisted around one another.
- Tension is created and portions of chromatid break off.
- These broken portions rejoin with chromatids of homologous partner.
- Usually equivalent portions of homologous chromosomes that are exchanged.
- In this way new genetic combinations are produced.
RECOMBINATION
Broken off pieces of chromosomes recombine with another chromatid.
